parser/lowering: declarative metasprites for multi-tile sprite groups

Multi-tile sprites used to require one hand-written `draw` per tile,
e.g. the four-call sequence in `examples/platformer.ne`'s
`draw_player()`. The new `metasprite Name { ... }` declaration
collects parallel `dx`/`dy`/`frame` arrays plus a reference to the
underlying sprite, and `draw Name at: (x, y)` expands to one OAM
slot per tile in the IR lowering — the codegen sees N regular
DrawSprite ops, so the runtime OAM cursor allocator picks them up
without any metasprite-specific awareness.

The metasprite's `frame:` array is interpreted *relative to the
underlying sprite's base tile*: index 0 means "the first tile this
sprite owns", which is the natural reading for a 16×16 hero whose
pixel art the asset resolver split into four consecutive tiles.
The lowering walks `program.sprites` to compute base tile indices
the same way `assets::resolve_sprites` would, then folds the base
into each frame entry before storing the metasprite info. Sprites
sourced from external `@chr(...)` / `@binary(...)` files whose
bytes aren't available at parse time fall back to a one-tile
assumption — those programs are rare and can declare metasprites
against pixel-art sprites instead.

The new `examples/metasprite_demo.ne` declares a 16×16 hero sprite
and arranges its four tiles into a metasprite, then sweeps the
hero across the screen so the harness captures it mid-motion.
The new keyword is added to the lexer/token list, and the parser
accepts `sprite:` (the otherwise-keyword) as a property name in
metasprite bodies so the natural spelling parses.

https://claude.ai/code/session_01KEczoNUX3WmcFLfq6iAQxB

README.md

@@ -91,6 +91,7 @@ start Main
 | [`audio_demo.ne`](examples/audio_demo.ne) | Audio subsystem: user `sfx`/`music` blocks, builtin effects, `play`/`start_music`/`stop_music` |
 | [`noise_triangle_sfx.ne`](examples/noise_triangle_sfx.ne) | Noise and triangle channel sfx via `channel: noise` / `channel: triangle` on `sfx` blocks |
 | [`sfx_pitch_envelope.ne`](examples/sfx_pitch_envelope.ne) | Per-frame pulse `pitch:` arrays — the audio tick walks the pitch envelope in lockstep with the volume envelope and writes `$4002` on every NMI for a frequency-sweeping siren tone |
+| [`metasprite_demo.ne`](examples/metasprite_demo.ne) | `metasprite Hero { sprite: ..., dx: [...], dy: [...], frame: [...] }` declarative multi-tile groups — `draw Hero at: (x, y)` expands to one OAM slot per tile so 16×16 sprites stop needing four hand-written `draw` statements |
 | [`platformer.ne`](examples/platformer.ne) | **End-to-end side-scroller** — custom CHR tileset, full background nametable, metasprite player with gravity/jump physics, wrap-around scrolling, stomp-or-die enemy collisions, live stomp-count HUD, pickup coins, user-declared SFX + music, and a Title → Playing → GameOver state machine with a proximity-based autopilot so the headless harness demonstrates the full gameplay loop (stomp, stomp, die, retry) inside six seconds |
 
 ## Compiler Commands