fix(tiles): early status on dictionary failure, decode-time field, chunk bound

docs/superpowers/specs/2026-10-30-vt1200-video-streaming-design.md

@@ -182,8 +182,9 @@ frame start, so a frame never straddles the ring wrap and slot ownership is expl
 * **Dictionary**: 256 × 32×32 1-bpp (128 B each = 32 KB), built offline by k-means
   over the video's tiles, uploaded in chunks, held in the startup allocation.
 * **Frame**: header + 1,280 index bytes (40×32 grid, row major). `kind = TILES_FULL`.
-* **Delta**: `kind = TILES_DELTA` + a changed-tile list (run-length list of tile
-  indices, 1–2 bytes per entry) and their new indices. Deltas are the normal case.
+* **Delta**: `kind = TILES_DELTA` + a changed-tile list: `count u16`, then
+  `count` × (`pos u16`, `index u8`) little-endian, where `pos` is the row-major
+  tile position and `index` its new dictionary entry. Deltas are the normal case.
 * Decoder: one cached loop; `PIXBLT B,XY` from `dict + index*128`, `DYDX=(32,32)`,
   `SPTCH=32` (a 32-px row is **two** words), destination XY `(ty*32, tx*32)`.
 * **Repair**: a `TILES_FULL` keyframe every 2 s (and on the host's first frame), so