rpg: add MVP-2 asset spec doc

Author: omiq

examples/rpg/asset-spec.md

@@ -0,0 +1,87 @@
+# MVP-2 Zelda-lite — asset spec
+
+Drop assets in this directory (`examples/rpg/`). Manifest format
+matches `examples/shooter/manifest.txt`. Once delivered, the engine
++ level files get scaffolded as `rpg.bas`, `level1.bas`,
+`level1_cave.bas`, all consuming `maplib.bas` (per
+`docs/map-format.md`).
+
+## Required
+
+### Tilesets — exercises multi-tileset support
+
+```
+overworld.png   256x128  cellW=32 cellH=32    32 tiles: grass, water, sand,
+                                              path, tree, rock, fence, sign,
+                                              flowers, etc.
+cave.png        256x128  cellW=32 cellH=32    32 tiles: stone floor, wall,
+                                              door, stairs, torch, chest,
+                                              cracked-wall variant.
+```
+
+### Player + NPCs + enemies
+
+```
+link.png        160x32   cellW=32 cellH=32    5 frames: idle + up + down +
+                                              left + right (one frame each;
+                                              animate later).
+npc.png         32x32                         single friendly NPC sprite
+                                              (old man / merchant).
+enemy.png       64x32    cellW=32 cellH=32    1 enemy, 2-frame walk anim
+                                              (slime / octorok).
+```
+
+## Optional (skip if shipping faster matters)
+
+```
+items.png       64x16    cellW=16 cellH=16    4 items: heart, key, sword,
+                                              gem (chest drops + HUD).
+heart.png       8x8                           HUD heart pip (lives display).
+effects.png     32x32                         sword-slash / sparkle one-shot.
+```
+
+## Per-tileset metadata to provide
+
+For each tileset, list the **local tile ids** that fall into each
+category. (Per the format spec, ids are 1-based local to each sheet;
+0 = blank.)
+
+### `overworld.png`
+
+- **solid** (block player + projectiles): _list ids_
+- **damaging** + damage value (lava / spikes — optional): _list ids_
+- **kind: door** — id of any tile that triggers map transition on
+  walk-into: _list ids_
+
+### `cave.png`
+
+- **solid**: _list ids_
+- **kind: door** (back to overworld): _list ids_
+- **kind: chest** (interactable, drops items): _list ids_
+
+## Design choices to confirm before scaffold
+
+| Decision | Default proposal |
+|---|---|
+| Overworld map size | 32 × 32 cells (1024 × 1024 px world) |
+| Room size | 8 × 8 cells → 4 × 4 grid = 16 rooms (Zelda 1 screen-flip) |
+| Cave map | separate file `level1_cave.bas`, 8 × 8 cells (single room) |
+| Door pairing | one overworld door → cave entry; cave stairs → back to overworld |
+| Combat | sword swipe on bump — kills enemy if facing it (no projectiles) |
+| NPC interact | walk-into-NPC opens dialogue text in HUD strip; E or SPACE dismisses |
+| Camera mode | `room` — snap to current 8×8 room on player crossing boundary |
+| HUD | top 24 px strip: hearts (lives) + dialogue text |
+
+## What this MVP validates in `docs/map-format.md`
+
+- **multi-tileset map** (`tilesets[]` length 2; layers reference
+  `tilesetId`)
+- **multi-layer rendering** (terrain + decoration + hidden collision)
+- **camera.mode = "room"** snap behaviour
+- **door pairing** via `props.leadsTo` + `props.spawnAt`
+- **objects** with `kind: npc | enemy | door | chest | spawn`
+- **per-instance state** vs static tile flag (player has key 5 lives
+  in savegame domain; tile.solid is static tileset metadata)
+
+Together with MVP-1 shooter, every concept in v1 of the format
+(except polygon shapes — explicitly deferred) gets exercised.