commit built ROMs alongside .ne sources

The compiler is deterministic: rebuilding any example produces
a byte-identical ROM, verified across all 22 examples and all
four mappers (NROM, MMC1, UxROM, MMC3). That means the .nes
files are reproducible artefacts and can live next to their
sources without drift.

Benefits:

- Users can clone the repo and open any example in an emulator
  without installing a Rust toolchain or running the compiler.
- The emulator harness can trust examples/*.nes directly, so its
  CI job no longer needs a compiler build or a "compile all
  examples" loop — it just boots jsnes against the committed
  ROMs and diffs each against its golden.
- ROM diffs in PRs are now meaningful: "this compiler change
  flipped 17 bytes in hello_sprite.nes" is visible review
  signal, not hidden behind the emulator golden.

Guard rails so the ROMs don't drift from their sources:

- .gitignore no longer excludes *.nes.
- The `examples` CI job rebuilds every .ne into /tmp and fails
  loudly (with a GitHub error annotation pointing at the exact
  cargo command to rerun) if any committed ROM differs.
- scripts/pre-commit does the same check locally.
- CLAUDE.md now states that editing a .ne file requires
  rebuilding its .nes in the same commit, so future agents
  won't miss the invariant.

Total footprint: 22 ROMs, 624 KB (avg 28 KB each — most are
NROM 24 KB; two banked examples are larger).

https://claude.ai/code/session_01BcCcHi6FUmTh8jC7UgkA3A

.github/workflows/ci.yml

@@ -58,12 +58,26 @@ jobs:
       - uses: dtolnay/rust-toolchain@stable
       - uses: Swatinem/rust-cache@v2
       - run: cargo build --release
-      - name: Compile all .ne examples
+      - name: Verify committed ROMs match their .ne sources
+        # Every example has a committed .nes file under examples/. The
+        # compiler is deterministic (verified: same input → byte-identical
+        # output), so rebuilding each example into a tmp directory and
+        # diffing against the committed ROM catches "someone edited a .ne
+        # file but forgot to regenerate the ROM" at PR time.
         run: |
+          mkdir -p /tmp/reprotest
+          fail=0
           for f in examples/*.ne; do
-            echo "==> Compiling $f"
-            cargo run --release -- build "$f"
+            name=$(basename "$f" .ne)
+            ./target/release/nescript build "$f" --output "/tmp/reprotest/$name.nes"
+            if ! cmp -s "examples/$name.nes" "/tmp/reprotest/$name.nes"; then
+              echo "::error file=examples/$name.ne::committed examples/$name.nes is stale; rerun \`cargo run --release -- build examples/$name.ne\` and commit the new ROM"
+              fail=1
+            fi
           done
+          if [ $fail -ne 0 ]; then
+            exit 1
+          fi
       - name: Verify ROMs are valid iNES
         run: |
           for f in examples/*.nes; do
@@ -83,8 +97,6 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
-      - uses: Swatinem/rust-cache@v2
       - uses: actions/setup-node@v4
         with:
           node-version: '22'
@@ -110,14 +122,9 @@ jobs:
             libxkbcommon0 \
             libxrandr2 \
             libasound2t64
-      - name: Build compiler
-        run: cargo build --release
-      - name: Compile all .ne examples
-        run: |
-          for f in examples/*.ne; do
-            echo "==> Compiling $f"
-            ./target/release/nescript build "$f"
-          done
+      # No compiler build needed — the `.nes` files are committed
+      # alongside their `.ne` sources, and the `examples` CI job
+      # fails on any stale ROM, so the emulator job can trust them.
       - name: Install emulator harness dependencies
         working-directory: tests/emulator
         run: npm ci