main: write the real mapper byte into the iNES header

The CLI's build path was calling `Linker::new(mirroring)`, which
hardcodes the mapper number to NROM (0) regardless of the source
file's `mapper:` declaration. That meant MMC1/MMC3 examples shipped
with the wrong mapper byte in their iNES header — jsnes and Mesen
both read the header to pick a board, so they were running the
MMC3 examples under NROM semantics (no scanline IRQ scheduling, no
PRG bank switching support, etc.). The Rust integration tests
already used `Linker::with_mapper` via `compile_with_mapper`, so
the unit-level MMC coverage was correct; only the CLI output was
wrong.

Swap to `Linker::with_mapper(program.game.mirroring, program.game.mapper)`
so the header matches the source. Confirmed by inspecting the
rebuilt example ROMs:

  mmc3_per_state_split.nes: flags6=40 (mapper=4)  ← was 00
  scanline_split.nes:       flags6=40 (mapper=4)  ← was 00
  mmc1_banked.nes:          flags6=11 (mapper=1)  ← was 01
  hello_sprite.nes:         flags6=00 (mapper=0)  unchanged

Under real MMC3 semantics jsnes now runs the scanline IRQ path
for the two scanline examples, which ends up producing 9 more
audio samples (~200 μs) in the 180-frame capture window — a
timing difference that falls out of how the IRQ handlers
interact with the audio frame counter. Updated the two audio
goldens to match: `a82b6ff5 132084` -> `e76240c5 132093` for
both `mmc3_per_state_split` and `scanline_split`. PNG goldens
are unchanged — the visible output is the same.

All 19 emulator goldens now match. 381 unit tests + 43 integration
tests green. Clippy and fmt clean.

https://claude.ai/code/session_015WfaDttE3DpWn9rpyfpQd8

src/main.rs

@@ -313,13 +313,12 @@ fn compile(input: &PathBuf, opts: &CompileOptions) -> Result<Vec<u8>, ()> {
     }
 
     // Link into ROM with both graphic assets (sprite CHR) and audio
-    // assets (sfx envelopes, music note streams) spliced in. Note:
-    // `Linker::new` hardcodes the mapper byte to NROM regardless of
-    // the source's `mapper:` declaration — that's a pre-existing
-    // bug separate from the audio work, and fixing it would change
-    // the iNES header of committed MMC1/MMC3 example ROMs. Left as
-    // `new` here to keep this PR focused on audio.
-    let linker = Linker::new(program.game.mirroring);
+    // assets (sfx envelopes, music note streams) spliced in. We use
+    // `Linker::with_mapper` so the iNES header's mapper byte
+    // reflects the source's `mapper:` declaration — without this
+    // the CLI always shipped mapper 0 (NROM) regardless of whether
+    // the program actually needed MMC1/MMC3 bank switching.
+    let linker = Linker::with_mapper(program.game.mirroring, program.game.mapper);
     let rom = linker.link_with_all_assets(&instructions, &sprites, &sfx, &music);
 
     Ok(rom)

tests/emulator/goldens/mmc3_per_state_split.audio.hash

@@ -1 +1 @@
-a82b6ff5 132084
+e76240c5 132093

tests/emulator/goldens/scanline_split.audio.hash

@@ -1 +1 @@
-a82b6ff5 132084
+e76240c5 132093