palette/background: first-class declarations with reset-time load and runtime swaps

Re-adds `palette Name { colors: [...] }` and
`background Name { tiles: [...], attributes: [...] }` as first-class
declarations, plus `set_palette Name` and `load_background Name`
statements for runtime swaps. Unlike the previous iteration that
quietly no-op'd, this one is fully wired through the pipeline and
its behavior is pinned by both unit tests and an emulator golden.

Pipeline:

- Lexer: re-adds `palette`, `background`, `set_palette`,
  `load_background` keywords and tokenizes them.
- AST: `PaletteDecl` (name + 1..=32 colour bytes) and `BackgroundDecl`
  (name + 0..=960 tile bytes + 0..=64 attribute bytes) live in
  `Program`. `Statement::SetPalette` and `Statement::LoadBackground`
  name-reference these declarations.
- Parser: `palette Name { colors: [...] }` / `background Name
  { tiles: [...], attributes: [...] }` blocks and their statement
  forms parse via the existing byte-array helper.
- Analyzer: validates colour indices ($00-$3F), palette length
  (<=32), nametable length (<=960), attribute length (<=64), and
  duplicate decl names. `set_palette` / `load_background` targets
  must reference a declared name (E0502 otherwise). When a program
  declares palette or background, the analyzer bumps the user
  zero-page allocator's starting address from `$10` to `$18` to
  reserve `$11-$17` for the runtime update handshake — programs
  that don't use the feature keep the old layout so their emulator
  goldens stay byte-exact.
- Assets: `PaletteData` and `BackgroundData` resolve declarations
  into zero-padded fixed-size blobs (32 / 960 / 64 bytes) and
  expose `label()` / `tiles_label()` / `attrs_label()` for codegen
  to reference.
- IR: new `IrOp::SetPalette(String)` and
  `IrOp::LoadBackground(String)`; lowering forwards the names
  verbatim.
- Codegen: `gen_set_palette` writes the palette label pointer into
  ZP `$12/$13` and ORs bit 0 into the update flags at `$11`;
  `gen_load_background` does the same for tile/attribute pointers
  at `$14/$15/$16/$17` with bit 1. Both emit a `__ppu_update_used`
  marker so the linker splices in the NMI apply helper only when
  the feature is actually used.
- Runtime: `gen_initial_palette_load` and
  `gen_initial_background_load` write the first declared
  palette/background at reset time (before rendering is enabled,
  where PPU writes are safe). `gen_nmi(has_ppu_updates)` takes a
  new flag; when true it splices `gen_ppu_update_apply` at the top
  of the NMI body, which checks the `$11` flags byte and copies
  pending palette / nametable data to `$3F00` / `$2000` inside
  vblank. All helpers use only ZP $02/$03 as scratch at reset time
  and never clobber ZP slots live across NMI.
- Linker: new `link_banked_with_ppu` takes slice of `PaletteData` /
  `BackgroundData`; splices each blob as a labelled data block in
  PRG ROM, picks the first-declared as the reset-time load target,
  enables background rendering automatically when a background is
  declared, and threads `has_ppu_updates` into `gen_nmi`. Old
  `link_banked` remains as a thin wrapper for callers without
  palette/background data so existing tests don't shift.

Tests:

- Lexer: tokenization of the 4 new keywords (single added test case).
- Parser: 5 new tests for `palette` / `background` decls with and
  without attributes, plus `set_palette` / `load_background`
  statements.
- Analyzer: 9 new tests covering acceptance of declared
  palettes/backgrounds, E0502 for unknown names, E0201 for
  out-of-range NES colors and oversized blobs, E0501 for duplicate
  names, and the zero-page-layout guard (palette/bg decls bump ZP
  start; no decls keeps it at $10).
- Resolver: 3 new tests for zero-padding, truncation of oversized
  decls, and label derivation.
- IR: 2 new lowering tests for `set_palette` and `load_background`.
- Integration: 5 new tests — blob contents spliced verbatim into
  PRG, `STA $12` / `STA $14` emitted by set_palette /
  load_background codegen, and a regression guard that programs
  without palette/background still land user vars at $10.
- Emulator: new `examples/palette_and_background.ne` driven by a
  frame counter that toggles between `CoolBlues` / `WarmReds` and
  `TitleScreen` / `StageOne` every 90 frames. Golden PNG and audio
  hash checked in under `tests/emulator/goldens/` and verified via
  `node run_examples.mjs` — rendered image shows the blue
  `CoolBlues` palette with the nametable populated from
  `TitleScreen`.

Docs:

- `README.md` adds the feature to the headline list and the example
  table.
- `docs/language-guide.md` restores the palette/background sections
  with the full 32-byte layout table and `set_palette` /
  `load_background` statement references.
- `docs/future-work.md` replaces the "removed as dead code" entry
  with the remaining gaps (PNG-sourced palette and nametable
  assets, cross-vblank large background updates, memory-map
  reporting).
- `spec.md` restores the grammar productions and usage examples.
- `examples/README.md` lists the new demo.

All 497 unit + integration tests pass. Clippy clean. All 21
emulator goldens match after the update pass.

https://claude.ai/code/session_012fKB251HvEUQwG3tizFyqt

docs/language-guide.md

@@ -641,6 +641,27 @@ Set the PPU scroll position:
 scroll(scroll_x, scroll_y)
 ```
 
+### Set Palette
+
+```
+set_palette NightPalette
+```
+
+Queues the named palette for a vblank-safe copy into PPU palette
+RAM (`$3F00-$3F1F`). The write is applied by the NMI handler on the
+next vblank. See `palette` declarations below.
+
+### Load Background
+
+```
+load_background Level1
+```
+
+Queues the named background (a full-screen 32×30 nametable + 64-byte
+attribute table) for a vblank-safe copy into nametable 0
+(`$2000-$23FF`). Applied by the NMI handler at the next vblank. See
+`background` declarations below.
+
 ### Function Calls as Statements
 
 ```
@@ -664,6 +685,72 @@ sprite Coin {
 }
 ```
 
+### Palette Declarations
+
+```
+palette MainPalette {
+    colors: [0x0F, 0x01, 0x11, 0x21,
+             0x0F, 0x06, 0x16, 0x26,
+             0x0F, 0x09, 0x19, 0x29,
+             0x0F, 0x0B, 0x1B, 0x2B,
+             0x0F, 0x01, 0x11, 0x21,
+             0x0F, 0x16, 0x27, 0x30,
+             0x0F, 0x14, 0x24, 0x34,
+             0x0F, 0x0B, 0x1B, 0x2B]
+}
+```
+
+Each byte is an NES master-palette index (`$00-$3F`). The 32 bytes
+map directly to PPU palette RAM `$3F00-$3F1F` in the canonical NES
+layout:
+
+| Offset          | Contents                            |
+|-----------------|-------------------------------------|
+| `$00`-`$03`    | Background sub-palette 0 (4 colours) |
+| `$04`-`$07`    | Background sub-palette 1             |
+| `$08`-`$0B`    | Background sub-palette 2             |
+| `$0C`-`$0F`    | Background sub-palette 3             |
+| `$10`-`$13`    | Sprite sub-palette 0                 |
+| `$14`-`$17`    | Sprite sub-palette 1                 |
+| `$18`-`$1B`    | Sprite sub-palette 2                 |
+| `$1C`-`$1F`    | Sprite sub-palette 3                 |
+
+The first byte of each sub-palette is typically `$0F` (black); it
+serves as the shared background colour. Lists shorter than 32 bytes
+are zero-padded; lists longer than 32 are a compile error.
+
+The *first* `palette` declared in a program is loaded into VRAM at
+reset time, before rendering is enabled, so the title screen boots
+with the right colours on frame 0. Additional declarations sit in
+PRG ROM as named data blobs and become active via `set_palette Name`,
+which queues the write for the next vblank.
+
+### Background Declarations
+
+```
+background TitleScreen {
+    tiles: [0x00, 0x01, 0x01, 0x00,  /* ... up to 960 bytes ... */]
+    attributes: [0xFF, 0x55,         /* ... up to 64 bytes ... */]
+}
+```
+
+A background is a 32×30 nametable. `tiles` is the nametable itself
+— 960 bytes of CHR tile indices, one per 8×8 cell, in row-major
+order (left-to-right, top-to-bottom). `attributes` is the 8×8
+attribute table (64 bytes) that controls which sub-palette each
+16×16 metatile uses.
+
+Both lists are zero-padded up to their fixed sizes; `attributes`
+may be omitted entirely, in which case every cell uses background
+sub-palette 0.
+
+The *first* `background` declared is loaded into nametable 0 at
+reset time and background rendering is enabled automatically.
+Additional backgrounds can be swapped in via `load_background Name`,
+which queues the update for the next vblank. Full-nametable updates
+do not fit inside a single vblank, so large background swaps may
+require the program to disable rendering temporarily.
+
 ### Asset Sources
 
 Three ways to provide asset data: