Follow-up cleanup on the cc65 parity batch. Addresses issues found
during a post-commit code review.
**Correctness fixes:**
- `rand8()` / `rand16()` at statement position (result discarded)
were being eliminated by DCE because `op_dest` returned
`Some(dest)` for Rand8/Rand16 even though the ops have a visible
side effect — advancing the PRNG state. Now `op_dest` returns
`None` for both, keeping the JSR regardless of liveness. New
regression test `rand8_statement_survives_dce`.
- Void-only intrinsics (`poke`, `seed_rand`, `set_palette_brightness`)
used in expression position (e.g. `var x = seed_rand(42)`) were
panicking the linker with an unresolved `__ir_fn_X` label. The
analyzer now emits E0203 with a clear message; new
`void_intrinsic_in_expression_position_errors` test covers all
three names.
- Statement-position `rand8()` / `rand16()` weren't lowered at all
(they fell through to the default Call path). Now both lower to
their IR op with a fresh temp that nothing reads; the JSR still
runs so the PRNG state advances.
- `--fceux-labels foo.nes` was producing `foo.0.nl` because
`PathBuf::with_extension` replaces instead of appends. Rewritten
to literally append `.<bank>.nl` / `.ram.nl` to the OsString, so
users get the FCEUX-expected `foo.nes.<bank>.nl` naming.
- Linker now asserts CNROM / AxROM don't accept user-declared
switchable PRG banks — their page sizes don't fit the 16 KB per
bank model, and silently producing a mis-sized ROM is worse than
a loud panic.
**PRNG cleanup:**
- Removed the stream-of-consciousness comment block in `gen_prng`
that described three abandoned algorithms before landing on the
actual Galois LFSR.
- Simplified `__rand16` to a single JSR + LDX instead of two
JSRs + TAY/TYA round-trip — a single shift already produces 16
fresh bits, the doubled call just burned ~40 cycles. The golden
PNG for `prng_demo` was regenerated to reflect the new sequence.
- Rewrote the `gen_prng` doc comment to accurately describe the
algorithm as a Galois LFSR (it was mislabelled as xorshift).
- Rewrote the `gen_palette_brightness` doc comment with a proper
table of level→mask mappings — the prior prose description
didn't match the actual table values.
**Tests:**
- Three new unit tests in `linker::debug_symbols` covering the
FCEUX `.nl` renderer: user-facing labels only, empty output when
no user labels exist, and deterministic sorting in `.ram.nl`.
- Extended `nes2_mapper_high_nibble_in_byte_8_is_zero_for_small_mappers`
to cover AxROM + CNROM.
- Renumbered priority list in future-work.md after removing the
shipped sections (J, K, N, parts of V and Y).
All 737 tests + 40/40 emulator goldens still green.
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.
