Improve meeting start failure diagnostics

[r3dbars]

Summary

• Add typed, privacy-safe meeting capture start stages (microphone_graph, system_audio, microphone_file, unknown) and use them to classify generic start failures.
• Preserve truthful system-audio failed state in start diagnostics and allowlist bounded route/VPIO fields for Sentry triage.
• Distinguish true Parakeet audio-engine timeouts from circuit-open fail-closed events without changing timeout, retry, or route policy.
• Reset typed failure state before preflight failures so a new attempt cannot inherit the previous attempt’s stage.

Verification

• bash build-deps.sh --force
• bash build.sh --no-open
• bash run-tests.sh — 12,239 passed
• bash run-integration-smoke.sh
• swift test — 1,039 passed, 13 skipped
• python3 scripts/ops/normalize-analytics-taxonomy.py --check
• python3 scripts/dev/check-build-source-lists.py
• git diff --check
• codex review --base origin/main — clean, no actionable findings

Proof boundary

Deterministic tests and signed app launch smoke pass. Real Bluetooth split-route/HFP capture, ScreenCaptureKit system-audio capture, and installed-client recovery remain manual hardware proof; no hardware route change is claimed or validated here.

This is a draft for review. No release, appcast, Homebrew, notarization, deployment, or merge was performed.