feat(canvas): 4096^2 canvas, 256-color palette (u8 byte-aligned), custom picker (#5)

Canvas:
- CANVAS_W/H = 4096, total 16,777,216 pixels
- BITS_PER_PIXEL = 8 (byte-aligned) — raw Redis bytes are palette indices
- Canvas-decoder becomes an identity wrap/copy
- Storage BITFIELD uses u8; offset = y*W + x
- Redis key versioned to rplace:canvas:v2 so old 32-color/2048^2 data is
  orphaned (operators can DEL the old key to reclaim memory)

Palette:
- 256 entries, generated deterministically:
  - 0..15  = 16-step grayscale ramp (pure black -> pure white)
  - 16..255 = 240 HSL wheel (4 lightness rings x 60 hues @ 82% saturation)
- nearestPaletteIndex(r,g,b) helper for custom-color snapping

UI:
- ColorPicker: 16-swatch favorites strip (grays + 8 accents) + current-color
  swatch + expand toggle for the full 16x16 grid + "Custom..." button that
  opens the native <input type="color"> and snaps to nearest palette entry
- Default selected color bumped to index 0 (black)

Tests + docs:
- canvas-decoder tests rewritten for identity u8 decode
- canvas-storage tests updated for u8 offsets
- image-to-palette tests anchored to PALETTE_BLACK=0 / PALETTE_WHITE=15 and
  COLORS_RGBA[i] probes (no more hardcoded old 32-color indices)
- integration test uses u8 BITFIELD and canvas-aware bounds
- README, system-architecture, deployment-guide updated (storage math,
  migration note for orphaned old key)

docs/deployment-guide.md

@@ -69,6 +69,7 @@ For hobby traffic (< few hundred users/day), free tiers are sufficient. Upstash
 ## Troubleshooting
 
 - **Canvas loads empty**: Check Upstash credentials in secrets
-- **Pixels don't persist**: Verify BITFIELD support — test with `redis-cli BITFIELD canvas SET u5 #0 1`
+- **Pixels don't persist**: Verify BITFIELD support — test with `redis-cli BITFIELD rplace:canvas:v2 SET u8 #0 42`
+- **Old 32-color canvas still visible after deploy**: the canvas key is versioned (`rplace:canvas:v2`). The old `rplace:canvas` key is orphaned — run `redis-cli DEL rplace:canvas` once to reclaim memory.
 - **WebSocket not connecting**: Ensure Durable Object migration ran (check `wrangler.json` migrations)
 - **Rate limiting not working**: Verify `SET key value NX EX 1` returns `"OK"` / `null` as expected on your Upstash tier

docs/system-architecture.md

@@ -23,9 +23,9 @@ rplace is a collaborative pixel canvas deployed as a single Cloudflare Worker. T
 
 ```
 1. Client fetches GET /api/canvas
-2. Worker reads Redis key via GETRANGE → raw binary
-3. Client receives ~2.5MB (5-bit packed pixels)
-4. Client decodes 5-bit values → color indices → RGBA ImageData
+2. Worker reads Redis key via GETRANGE → raw binary (16 MB, gzip-compressed by CF edge)
+3. Client receives 16 MB of bytes — each byte is a palette index (u8, byte-aligned)
+4. Client maps indices → RGBA ImageData via COLORS_RGBA lookup
 5. Renders onto HTML5 Canvas with OffscreenCanvas
 ```
 
@@ -42,13 +42,14 @@ rplace is a collaborative pixel canvas deployed as a single Cloudflare Worker. T
 
 ## Storage
 
-### Redis BITFIELD (Canvas)
+### Redis STRING / BITFIELD (Canvas)
 
-- Key: `canvas`
-- Encoding: 5 bits per pixel (u5), 32 colors
-- Size: `2048 * 2048 * 5 / 8 = 2,621,440 bytes` (~2.5MB)
+- Key: `rplace:canvas:v2` (bumped from the old `rplace:canvas` so the 32-color/2048² data is ignored on rollout)
+- Encoding: 8 bits per pixel (u8), 256-color palette — byte-aligned, so raw Redis bytes are the pixel indices directly
+- Size: `4096 × 4096 × 1 = 16,777,216 bytes` (16 MB)
 - Offset: `y * CANVAS_WIDTH + x`
-- Atomic batch writes: single BITFIELD command with chained .set() calls
+- Atomic batch writes: single BITFIELD command chaining `SET u8 #offset color` per pixel
+- Reads via GETRANGE return the whole buffer; Cloudflare edge handles gzip
 
 ### Redis STRING (Cooldown)