Dual Format: verify dual-canvas passthrough + fix EB/VOD save-before-launch

[Soulhackzlol]

What

Two related pieces of work that came out of investigating Twitch Dual Format (vertical video) support, tracked in #27.

1. Dual Format already works — add regression tests (test: commit)

Dual Format sends a horizontal + vertical canvas over one Enhanced Broadcasting RTMP connection. It turned out to need no production code changes: canvas_index lives only in the GetClientConfiguration JSON, and on the wire the vertical canvas is just additional TrackIds that the existing pass_through_multitrack path already forwards bit-faithfully.

Verified in production against real Twitch: landscape ×4 + portrait ×3 tracks received, HEVC vertical primary, multiple delay cuts stable, dashboard reported "Excellent". Also confirmed it composes with VOD audio + EB (separate pass_through_multitrack_audio flag).

New tests in src/h264.rs:

• select_video_bytes_forwards_dual_canvas_to_twitch
• select_video_bytes_keeps_horizontal_drops_vertical_for_non_twitch
• is_primary_video_idr_anchors_dual_format_cuts_to_horizontal_canvas — pins the one invariant (cuts anchor to the horizontal canvas) that a future canvas-aware cut would have to revisit.

Test count bumped 197 → 200 (EN + ES READMEs).

2. Fix EB/VOD save-before-launch footgun (fix: commit)

The "Launch OBS for EB + VOD" button's visibility is driven by the live VOD checkbox, but VOD only takes effect on save — saving is what writes OBS's EnableCustomServerVodTrack gate (user.ini) and flips InstantClone's VOD-track forwarding. Clicking Launch on unsaved state spawned OBS with EB but no VOD, a silent mismatch.

launchObsWithEb() now persists the form first and only launches if the save succeeded; saveDestForm() returns a success boolean for that gate.

Testing

• cargo test --release → 200 passed, 0 failed
• cargo fmt --check clean, cargo clippy --release --all-targets clean
• Dual Format forwarding verified live against Twitch (see above)

Closes #27.