fix(ppu): the field flag toggles every frame, not only when interlaced

[doublegate]

A v1.29.0 prerequisite, found where the plan said it would be.

The finding

Ppu::field is $213F bit 7. The code toggles it unconditionally at frame end
(end_of_scanline), which is what the hardware does. Its doc comment said:

Interlace field (toggles each frame when interlace is on).

That describes neither. Only the flag's use is interlace-conditional — interlace consumes it in
render.rs to select the odd/even row — and $213F exposes it regardless.

Why it is not cosmetic

The v1.29.0 dot-model work (B2.02 short line / B2.03 long line) needs a gate that keys on the
field. Reading the comment rather than the code makes that gate look unreachable in progressive
mode, which would have sent the implementation down a wrong path. The plan flagged exactly this
("field toggles unconditionally at frame end, so the gate is reachable — but field's doc comment
says otherwise and is stale against the code"); this confirms it against the source and pins it.

Verification

New test the_field_flag_toggles_every_frame_even_in_progressive_mode asserts io.interlace is
false, ticks four frames, and requires $213F bit 7 to read [0x80, 0x00, 0x80, 0x00].

Injection-checked rather than assumed: replacing the toggle with if self.io.interlace { ... } —
the behaviour the old comment described — fails the test with a constant [0, 0, 0, 0]. So the test
pins the mechanism, not the wording.

docs/ppu.md gains the rule in the same change, per the chip-doc requirement.

Gates: cargo test -p rustysnes-ppu, clippy -D warnings, fmt --check, and the no_std
thumbv7em-none-eabihf build all clean (the test uses a fixed array, not Vec, since the crate is
no_std).

🤖 Generated with Claude Code

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📥 Commits

Reviewing files that changed from the base of the PR and between c20dd02 and d332947.

📒 Files selected for processing (4)

• CHANGELOG.md
• crates/rustysnes-ppu/src/lib.rs
• docs/accuracysnes-plan.md
• docs/ppu.md

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[github-actions]

Antigravity review (Gemini via Ultra)

This PR corrects doc comments and docs/ppu.md to document that $213F bit 7 toggles unconditionally every frame, while adding a test to pin the existing implementation behavior.

Blocking issues

None found.

Suggestions

• CHANGELOG.md: Remove the unrelated "App Store §4.7 self-audit done (v1.30.0)" changelog entry to maintain a single logical change per PR/commit.
• Commit message / PR title: The title uses fix(ppu), but no runtime logic was modified in rustysnes-ppu (only docs, comments, and tests). docs(ppu) or test(ppu) is more accurate.

Nitpicks

• crates/rustysnes-ppu/src/lib.rs:1337: The test assumes field is initialized to false prior to the first frame completion. Explicitly checking p.read_reg(0x213F) & 0x80 == 0 before entering the loop would clarify why seen[0] expects 0x80.

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