Closes seven of the cc65/nesdoug parity gaps catalogued in
docs/future-work.md in a single pass. All of the new features are
gated on marker labels so programs that don't use them produce
byte-identical ROM output (every pre-existing committed .nes file
round-trips unchanged).
Language / runtime additions:
- `rand8()` / `rand16()` / `seed_rand(u16)` intrinsics backed by a
16-bit Galois LFSR (~30 bytes of runtime, ~40 cycles per draw).
Reset path seeds state to 0xACE1 so the first draw is useful even
without explicit seeding.
- `p1.button.a.pressed` / `.released` edge-triggered input via a
new ReadInputEdge IR op plus an NMI-side prev-frame snapshot into
$07E6/$07E7, gated on the `__edge_input_used` marker.
- `set_palette_brightness(level)` builtin mapping levels 0..8 to
PPU mask emphasis bytes (`$2001`) for neslib-style screen fades.
- `mapper: AxROM` (iNES 7) with automatic 32 KB PRG padding so
emulators that enforce mapper-7's 32 KB page size boot cleanly.
- `mapper: CNROM` (iNES 3) with a reset-time CHR bank 0 select.
- `--fceux-labels <prefix>` CLI flag emitting per-bank `.nl` label
files and a `.ram.nl` file for FCEUX's debugger.
Tests + examples:
- Five new example programs with committed .nes ROMs and
pixel+audio goldens: prng_demo, edge_input_demo,
palette_brightness_demo, axrom_simple, cnrom_simple.
- Seven integration tests covering JSR emission, the
omitted-when-unused invariant, the NMI prev-input snapshot, the
correct mapper numbers for AxROM/CNROM, and negative tests for
unknown button names and bad rand8 arity.
- `is_intrinsic()` now runs in expression-position Call paths too,
so `var x = rand8(1, 2)` errors at compile time instead of
silently dropping the extra arguments.
