compiler: i16 / SRAM saves / inline-asm dot labels / docs

Another batch from the cc65/nesdoug catalogue. All gated on
parser-level opt-in or default-false attributes so existing
programs produce byte-identical ROMs (no committed .nes file
changed).

**§A — `i16` signed 16-bit type:**
- New `KwI16` lexer token, `NesType::I16` AST variant, parser
  case in `parse_type`. Type-size and integer-type tables
  treat `i16` like `u16` (2 bytes, integer).
- IR lowering accepts `i16` everywhere it accepts `u16` for
  wide-load / wide-store / widen-narrow paths.
- New constant fold for `UnaryOp::Negate(IntLiteral(v))` that
  emits the wide two's-complement form. Without it, `var vy:
  i16 = -10` would zero-extend to `$00F6` (= 246) instead of
  sign-extending to `$FFF6` (= -10). Negative literals now
  store the right bytes.
- Comparisons reuse the existing unsigned 16-bit compare ops
  (matching the existing `i8` behaviour). Documented in the
  `NesType::I16` doc comment and in `future-work.md` §A.
- Example `examples/i16_demo.ne` with committed golden.
- Tests cover the literal-fold sign-extension and end-to-end
  compile of the example.

**§S — SRAM / battery-backed saves:**
- New `save { var ... }` top-level block. Lexer + parser opt
  into a dedicated `KwSave` token. Analyzer allocates save
  vars from a separate `next_sram_addr` bump pointer starting
  at `$6000`, capped at `$8000` (8 KB cartridge SRAM window).
- Linker reads `analysis.has_battery_saves` and flips iNES
  byte-6 bit-1 via the new `RomBuilder::set_battery` /
  `Linker::with_battery` chain.
- New `W0111` warning for save-var initializers — SRAM is
  preserved across power cycles, so an init expression would
  either silently never run or clobber persisted data on
  every boot. The warning teaches the user about the
  magic-byte sentinel pattern.
- Struct fields in save blocks are explicitly rejected for now
  (the field-flattening path uses the main-RAM allocator).
- Example `examples/sram_demo.ne` with committed golden, plus
  4 integration tests.

**§D (partial) — inline-asm `.label:` syntax:**
- Codegen-side mangler rewrites `.IDENT` → `__ilab_<N>_IDENT`
  per inline-asm block, where `<N>` is the call site's
  monotonic suffix. Two `asm { .loop: ... }` blocks in the
  same function now coexist without colliding in the linker's
  label table.
- Bounds checks on `.` placement: `$2002` and `name.field`
  are unaffected; only `.IDENT` in label / branch context
  triggers the rewrite. Two integration tests pin the
  uniqueness and dollar-vs-dot disambiguation.

**§X follow-up — Mesen trace-log docs:**
- New "Debugger-assisted workflows" section in
  `docs/nes-reference.md` walking through the Mesen / FCEUX
  log workflows alongside the new `debug_port:` attribute.

**Misc:**
- `future-work.md` updated to mark the shipped items out of
  the catalogue and reshuffle the priority ranking. Remaining
  niche follow-ups (signedness on Cmp16, struct save fields,
  inline-asm format specifiers) documented inline so future
  passes know the design.

All 757 tests pass. Clippy clean. 46/46 emulator goldens match.

examples/README.md

@@ -50,6 +50,8 @@ Open any `.nes` file in an NES emulator ([Mesen](https://www.mesen.ca/), [FCEUX]
 | `auto_sprite_flicker.ne` | `game { sprite_flicker: true }` | The `game` attribute equivalent of calling `cycle_sprites` at the top of every `on frame` handler. Same 12-sprite layout as `sprite_flicker_demo.ne`, minus the explicit call — the IR lowerer injects the op automatically when the flag is set, so it's byte-identical to a hand-rolled version without the per-site boilerplate. |
 | `fade_demo.ne` | `fade_out(n)`, `fade_in(n)` | Blocking fade helpers that walk brightness 4 → 0 and 0 → 4 with `n` frames per step. The runtime splices `__fade_out` / `__fade_in` plus a callable `__wait_frame_rt` helper when the builtin is used; fade use also forces `__set_palette_brightness` to be linked in since the fade body JSRs into it. |
 | `sprite_0_split_demo.ne` | `sprite_0_split(x, y)` | Mid-frame scroll change driven by the PPU's sprite-0 hit flag (`$2002` bit 6), so the effect works on any mapper — NROM, UxROM, MMC1 — not just MMC3 via `on_scanline(N)`. Two-phase busy-wait (wait for clear, then wait for set) guarantees the hit we're responding to came from the current frame. Requires a sprite in OAM slot 0 that overlaps opaque background pixels; this demo uses a full smiley background so every frame's sprite-0 hit fires deterministically. |
+| `i16_demo.ne` | `i16` signed 16-bit type | Negative literals fold to wide two's complement (`-10` → `$FFF6`), so `var vy: i16 = -10` stores the right bytes instead of the zero-extended `$00F6`. Comparisons currently use the unsigned 16-bit compare path (matching existing `i8` behaviour) — fine for positive ranges, wrong for negative compares. The companion `i16_negative_literal_sign_extends_to_wide_store` integration test guards the literal-fold path. |
+| `sram_demo.ne` | `save { var ... }` | Battery-backed save block. The analyzer allocates `high_score` and `coins` at `$6000+` (cartridge SRAM window) instead of main RAM, and the linker flips iNES header byte-6 bit-1 so emulators (FCEUX, Mesen, Nestopia) load and persist the region from a `.sav` file alongside the ROM. SRAM is uninitialized at first power-on; production games should reserve a magic-byte sentinel and validate it before trusting the rest of the data — the compiler doesn't auto-initialize and emits W0111 if you try. |
 
 ## Emulator Controls