feat(hero): Phase 3 — media slot and split-layout variants

- New `content.media` field accepts image (Hugo-pipeline-processed for
  responsive srcset) or video (passes through as <video>). Optional
  `dark_src` provides a dark-mode image variant.
- New `design.layout` enum: centered (default) | split-left |
  split-right | stacked. Split layouts auto-set alignment to 'left'
  unless explicitly overridden.
- Bump trust-strip star size from w-4 to w-5 for visual presence.
- Two new shared partials in layouts/_partials/functions/:
  - resolve_image_resource: path → {resource | fallback_url}
  - process_image_for_props: path → {src, srcset, width, height}
  These factor out the resource-resolution + responsive-processing
  pipeline so media-bearing blocks don't have to inline it.
- Capabilities registry: media-image, media-video, split-layouts.
- Landing page _index.md: keeps the centered hero (no behavioural
  change) but documents the opt-in pattern for split layouts inline.

modules/blox/schemas/blocks.json

@@ -89,7 +89,10 @@
             "announcement-badge",
             "trust-strip",
             "size-presets",
-            "alignment"
+            "alignment",
+            "media-image",
+            "media-video",
+            "split-layouts"
           ],
           "complexity": "simple",
           "preact": true,