fix: canceled STT uploads, note autosave races, exports, and search

[MyButtermilk]

Scriber could upload canceled recordings, lose a reverted Meeting note during shutdown, truncate exported Meeting duration, and return unrelated history entries for a percent-sign search.

Fixes

• Discard buffered audio on CancelFrame across the shared cloud adapter, AssemblyAI, Mistral, Modulate, Smallest, and Soniox. Release adopted Rust WAV artifacts without opening or uploading them. Close recording-gate state without emitting a VAD stop that would commit canceled audio. Normal End/Stop finalization still runs once.
• Compare Meeting note teardown saves against the newest effective write generation, preserving user reverts even when older autosaves or newer teardown requests are in flight.
• Use the retained playback mix's full duration and Meeting-clock origin for exports, including trailing silence; retain segment-time fallback when playback duration is unavailable.
• Treat punctuation-only history queries as literal substrings so % cannot act as a SQL wildcard.

Each failure was reproduced against the original implementation before applying the fix. Changes are split into four commits, with durable contracts updated in AGENTS.md.

Validation

All six Hybrid PR Checks passed for final head 109fa074744f4994344f3a7bd5a423a6ab2a36a2.

• Windows Python full suite: 4,247 passed, 5 skipped.
• Rust: 303 passed; formatting and Clippy passed.
• Frontend: 109 library tests and 66 component tests passed; application/test TypeScript checks, ESLint, and production build passed.
• Real-browser File upload smoke passed against React/Vite and the production Python application composition.
• Extended Python mypy: no issues in 77 source files.
• Repository-wide Python Ruff, focused hybrid gates, QuickJS cache verification, and GitHub Actions syntax passed.
• Independent review found no blocking correctness issues. The configured Codex reviewer left a thumbs-up; no open review threads remain.
• Published Git tree hashes match the locally validated trees exactly.

The five Python skips concern optional locked QuickJS/FFmpeg and shipping-native-runtime qualification conditions. Provider regressions use local PCM and controlled upload callbacks; no live provider calls, physical microphone tests, installed-app smoke, installer build, or release were performed. The browser smoke ends at the durable queued-job boundary.

CI evidence

1. https://github.com/MyButtermilk/Scriber/actions/runs/33949478175