feat(frontend): wire the View -> Hide Overscan toggle

[doublegate]

Summary

Phase A.3 of the new UI/UX-parity plan (RustySNES ↔ RustyNES).

• View → Hide Overscan crops the trailing "overscan" scanlines a real 4:3 CRT wouldn't reliably
  show. The SNES's own SETINI register (rustysnes_ppu) already distinguishes the standard
  224-line display from a game-opted-in 239-line extension; app.rs's new crop_overscan crops
  exactly that extra 15-line extension back off.
• Applied once per frame at the point where both the synchronous and emu-thread render paths
  converge on the bytes actually being presented — after HD-pack compositing, run-ahead, and the
  PresentBuffer handoff have all already run, so it covers both build variants correctly.
• Crops a fraction (15/239) of the current height rather than a fixed pixel count, so it
  stays exact under an HD-pack integer upscale too (239 * scale * 15 / 239 reduces to exactly
  15 * scale, no rounding).
• Presentation-only (never touches the deterministic core), additive, false by default —
  byte-identical to every prior release when unchanged.

Test plan

• cargo test --workspace --exclude rustysnes-android — all green, including 3 new real unit
  tests (native resolution, HD-pack-scaled resolution, kept-bytes-untouched)
• cargo clippy --workspace --exclude rustysnes-android --all-targets -- -D warnings — clean
• cargo clippy -p rustysnes-frontend --features full -- -D warnings — clean
• cargo clippy -p rustysnes-frontend --no-default-features --features wasm-winit,emu-thread — clean
• cargo clippy -p rustysnes-frontend --target wasm32-unknown-unknown --features wasm-winit -- -D warnings — clean (only a known, pre-existing, unrelated save_states.rs finding remains, confirmed present on a clean origin/main checkout too)
• cargo fmt --check — clean
• RUSTDOCFLAGS="-D warnings" cargo doc --workspace --exclude rustysnes-android --no-deps — clean

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request implements the 'Hide Overscan' feature to crop the trailing 15 scanlines of the SNES's extended 239-line display back to 224 lines on the presentation side, including configuration options, UI integration, documentation, and unit tests. The reviewer identified a state desynchronization bug where capturing overscan_active before emulation runs can cause visual glitches during resolution transitions. To resolve this, the reviewer suggested checking dims.1 % 239 == 0 directly at the presentation site and provided code suggestions to update the check and its documentation.

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[Copilot]

Pull request overview

This PR implements the frontend wiring for View → Hide Overscan, a presentation-only option that crops the SNES 239-line “overscan” mode down to 224 lines at the point where framebuffer bytes are finalized for display (covering both synchronous and emu-thread render paths).

Changes:

• Add video.hide_overscan to the frontend config (default false) and expose it in the View menu UI.
• Apply overscan cropping during the final present-path framebuffer handling via a new crop_overscan helper and add unit tests for native + HD-pack-scaled cases.
• Document the feature in docs/frontend.md and add an Unreleased changelog entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
docs/frontend.md	Documents the new View → Hide Overscan behavior and its determinism boundary.
crates/rustysnes-frontend/src/ui_shell.rs	Adds the View menu checkbox that toggles cfg.video.hide_overscan.
crates/rustysnes-frontend/src/config.rs	Introduces VideoConfig::hide_overscan with docs and default false.
crates/rustysnes-frontend/src/app.rs	Implements the crop and wires it into the present path; adds unit tests.
CHANGELOG.md	Adds an Unreleased entry describing the new overscan-crop toggle.