feat(gallery): sidecar files, EXIF/IPTC, and page-bundle auto-discovery

Adds the metadata pipeline that makes the gallery block self-curating.
Per-image metadata now resolves through a four-tier waterfall, first
non-empty source wins:

  1. Frontmatter (items[].caption etc.)
  2. Sidecar file next to the image - tries six patterns:
     <name>.yaml / .yml / .json and <basename>.yaml / .yml / .json,
     looked up in both site assets and the page bundle (covers album
     mode and page-resource mode alike).
  3. Embedded IPTC/EXIF via Hugo's .Exif.Tags - caption from
     Caption-Abstract / ImageDescription / XPSubject / Description,
     credit from By-line / Artist / Creator. Wrapped in `try` so
     non-EXIF formats and bad metadata never break the build.
  4. Humanised filename - alt fallback only; captions stay empty
     rather than fabricated from filenames.

Also adds a third source mode - content.page_resources: true -
auto-discovers image resources attached to the current page bundle.
Drop-in for blog posts and project pages where photos live next to
index.md; sidecar lookup works inside the bundle too.

Fixes a leading-slash quirk: Resource.Name returns "/media/..." for
site assets but a bare path for page-bundle resources; the path is
now normalised so sidecar lookups land in either layout.

Test site adds two sidecar YAML files (04-cabin.jpg.yaml and
06-mountain.jpg.yaml) - the masonry album demo now picks up titles,
markdown captions, and links from those sidecars without listing any
items[].

modules/blox/layouts/_partials/functions/resolve_gallery_items.html

@@ -1,21 +1,27 @@
 {{/*
   resolve_gallery_items — Resolves gallery block items into a list of fully-processed
-  image props ready for the Preact component. Handles two source modes:
+  image props ready for the Preact component. Three source modes:
 
-    1. content.items[]  — explicit per-image dicts with optional caption/alt/credit/link
-    2. content.album    — folder path relative to assets/, e.g. "media/albums/gallery"
+    1. content.items[]        — explicit per-image dicts (curated)
+    2. content.album          — folder under assets/, e.g. "media/albums/gallery"
+    3. content.page_resources — auto-discover image resources in the current page bundle
 
   For each resolved image, produces a dict with:
     {src, srcset, width, height, aspect_ratio, lqip, alt, caption, credit, link, title}
 
-  Slice 1 caption resolution: frontmatter only (items[].caption). Sidecar + EXIF/IPTC
-  resolution is intentionally deferred to Slice 2 — for now, missing captions stay
-  empty in the UI, with the humanised filename surfaced as the `alt` fallback so
-  screen readers and search engines have something useful.
+  Metadata waterfall — first non-empty value per field wins:
+    1. Frontmatter (items[].caption etc.)
+    2. Sidecar file next to the image (.yaml / .yml / .json)
+         — tried as "<full-name>.yaml" then "<basename-without-ext>.yaml"
+         — looked up in both site assets and the current page bundle
+    3. Embedded IPTC/EXIF via Hugo's .Exif.Tags
+         — caption from Caption-Abstract / ImageDescription / XPSubject / Description
+         — credit from By-line / Artist / Creator
+    4. Humanised filename (alt only — captions stay empty when no metadata exists)
 
   Inputs:
-    .content: the block's content dict (items / album / sort / limit)
-    .Page:    current page (for page-resource image lookup)
+    .content: the block's content dict
+    .Page:    current page (used for page-resource lookup + RenderString markdown)
 
   Returns: dict { items: [...] } or empty if nothing resolves.
 */}}
@@ -27,7 +33,7 @@
 {{ $sort_order := lower ($content.sort | default "asc") }}
 {{ $limit := $content.limit | default 0 }}
 
-{{/* Build a unified list of {resource?, fallback_url?, meta} entries from either source mode */}}
+{{/* Build a unified list of {resource?, fallback_url?, meta} entries */}}
 {{ $candidates := slice }}
 
 {{/* Source A — explicit items */}}
@@ -61,6 +67,23 @@
   {{ end }}
 {{ end }}
 
+{{/* Source C — page bundle auto-discovery */}}
+{{ if $content.page_resources }}
+  {{ with $page }}
+    {{ $imgs := .Resources.ByType "image" }}
+    {{ $sorted := sort $imgs "Name" $sort_order }}
+    {{ range $sorted }}
+      {{ if in $allowed_exts (lower (path.Ext .Name)) }}
+        {{ $candidates = $candidates | append (dict
+          "resource" .
+          "fallback_url" ""
+          "meta" dict
+        ) }}
+      {{ end }}
+    {{ end }}
+  {{ end }}
+{{ end }}
+
 {{/* Apply limit */}}
 {{ if gt $limit 0 }}
   {{ $candidates = first $limit $candidates }}
@@ -73,17 +96,88 @@
   {{ $resource := .resource }}
   {{ $fallback_url := .fallback_url }}
 
-  {{/* Per-item text fields. Caption / title / credit are rendered through
-       Hugo's markdown pipeline at build time (inline mode — no wrapping <p>)
-       so users can put **bold**, *italic*, `code`, and [links](url) in any of
-       them without shipping a JS markdown parser. Alt stays as plain text
-       since it lives inside an HTML attribute. */}}
+  {{/* ---- Metadata waterfall ---- */}}
   {{ $alt := index $meta "alt" | default "" }}
   {{ $caption_raw := index $meta "caption" | default "" }}
   {{ $title_raw := index $meta "title" | default "" }}
   {{ $credit_raw := index $meta "credit" | default "" }}
   {{ $link := index $meta "link" | default "" }}
 
+  {{/* Tier 2 — sidecar file lookup. Tries:
+        <name>.yaml/.yml/.json (full filename, Lightroom-style convention)
+        <basename>.yaml/.yml/.json (filename minus extension)
+       in both site assets and the page bundle. */}}
+  {{ $sidecar := false }}
+  {{ if $resource }}
+    {{/* Resource.Name comes back with a leading slash for site assets (e.g.
+        "/media/albums/gallery/01.jpg"); strip it so resources.Get treats the
+        path as relative to assets/. Page-bundle resource names have no
+        leading slash, so TrimPrefix is a no-op for those. */}}
+    {{ $name := strings.TrimPrefix "/" $resource.Name }}
+    {{ $basename := strings.TrimSuffix (path.Ext $name) $name }}
+    {{ $patterns := slice
+      (printf "%s.yaml" $name)
+      (printf "%s.yml" $name)
+      (printf "%s.json" $name)
+      (printf "%s.yaml" $basename)
+      (printf "%s.yml" $basename)
+      (printf "%s.json" $basename)
+    }}
+    {{ range $pattern := $patterns }}
+      {{ if not $sidecar }}
+        {{ $candidate := resources.Get $pattern }}
+        {{ if not $candidate }}{{ $candidate = resources.GetMatch $pattern }}{{ end }}
+        {{ if and (not $candidate) $page }}
+          {{ $candidate = $page.Resources.GetMatch $pattern }}
+        {{ end }}
+        {{ if $candidate }}
+          {{ $sidecar = $candidate }}
+        {{ end }}
+      {{ end }}
+    {{ end }}
+  {{ end }}
+
+  {{ if $sidecar }}
+    {{ $data := transform.Unmarshal $sidecar }}
+    {{ if reflect.IsMap $data }}
+      {{ if not $caption_raw }}{{ $caption_raw = index $data "caption" | default "" }}{{ end }}
+      {{ if not $title_raw }}{{ $title_raw = index $data "title" | default "" }}{{ end }}
+      {{ if not $credit_raw }}{{ $credit_raw = index $data "credit" | default "" }}{{ end }}
+      {{ if not $alt }}{{ $alt = index $data "alt" | default "" }}{{ end }}
+      {{ if not $link }}{{ $link = index $data "link" | default "" }}{{ end }}
+    {{ end }}
+  {{ end }}
+
+  {{/* Tier 3 — embedded IPTC/EXIF. Lightroom/Capture One JPEG exports carry
+       Caption-Abstract + By-line + Copyright; this picks them up automatically. */}}
+  {{ $exif_tags := dict }}
+  {{ if $resource }}
+    {{ $exif_try := try $resource.Exif }}
+    {{ if and (not $exif_try.Err) $exif_try.Value }}
+      {{ $exif_tags = $exif_try.Value.Tags | default dict }}
+    {{ end }}
+  {{ end }}
+
+  {{ if $exif_tags }}
+    {{ if not $caption_raw }}
+      {{ range $key := (slice "Caption-Abstract" "ImageDescription" "XPSubject" "Description") }}
+        {{ if not $caption_raw }}
+          {{ $v := index $exif_tags $key }}
+          {{ if $v }}{{ $caption_raw = printf "%v" $v }}{{ end }}
+        {{ end }}
+      {{ end }}
+    {{ end }}
+    {{ if not $credit_raw }}
+      {{ range $key := (slice "By-line" "Artist" "Creator") }}
+        {{ if not $credit_raw }}
+          {{ $v := index $exif_tags $key }}
+          {{ if $v }}{{ $credit_raw = printf "%v" $v }}{{ end }}
+        {{ end }}
+      {{ end }}
+    {{ end }}
+  {{ end }}
+
+  {{/* ---- Inline-Markdown render of text fields (RenderString display:inline) ---- */}}
   {{ $caption := $caption_raw }}
   {{ if and $caption_raw $page }}
     {{ $caption = $page.RenderString (dict "display" "inline") $caption_raw }}
@@ -135,10 +229,8 @@
       ) }}
     {{ end }}
 
-    {{/* Alt fallback: plainified caption → humanised filename. Caption itself
-        is NOT filled from filename — empty captions stay empty in the UI
-        (Slice 1 behaviour). Plainify strips the HTML produced by RenderString
-        so alt stays attribute-safe. */}}
+    {{/* Tier 4 — alt fallback: plainified caption → humanised filename. Caption
+        itself stays empty when no metadata source has supplied one. */}}
     {{ if not $entry.alt }}
       {{ $fallback_alt := "" }}
       {{ with $caption_raw }}

test/assets/media/albums/gallery/04-cabin.jpg.yaml

@@ -0,0 +1,4 @@
+title: "The cabin"
+caption: "*Winter quarters* — where the team writes up over the slow months"
+credit: "Photo: [Picsum](https://picsum.photos)"
+alt: "Snow-dusted wooden cabin among pine trees"

test/assets/media/albums/gallery/06-mountain.jpg.yaml

@@ -0,0 +1,5 @@
+title: "Summit"
+caption: "Final ridge of the **Highland traverse**, with the trig point in view"
+credit: "Photo: [Picsum](https://picsum.photos)"
+link: "/expeditions/highland-traverse/"
+keywords: [highland, traverse, summit]