Fix transcription model startup and DMG signing

[daniellee-ux]

Summary

• Load complete WhisperKit model and tokenizer caches directly from disk, avoiding unnecessary network access during startup.
• Preserve model-load failures, expose correct Retry/Dismiss behavior, and make rapid or deferred model changes latest-selection-wins.
• Sign and strictly validate the DMG before notarization.
• Resolve one exact Developer ID SHA-1 for the selected Team and reuse it for archive, export, and DMG signing.
• Add transcription lifecycle/cache tests and Bash 3.2 signing-identity regression tests in CI.

Root causes

WhisperKit's download entry point can contact Hugging Face before consulting its cache, so an already-installed model could still appear to download, stall, or fail offline. Model setup errors were also cleared by recording-state reset, leaving the UI stuck in a loading state.

The packaging script passed the partial identity name Developer ID Application to standalone codesign. With more than one matching certificate installed, codesign fails because it does not use Xcode's DEVELOPMENT_TEAM setting to disambiguate.

Validation

• Focused transcription tests: 11/11 passed.
• Full macOS test suite: 421/421 passed.
• Release build passed.
• Exact-SHA Developer ID probe and signed Release archive verification passed; TeamIdentifier matched.
• Bash 3.2 syntax and signing-identity fixture tests passed.
• Workflow YAML and diff checks passed.
• Multiple independent read-only reviews completed; all earlier actionable findings were fixed.

[daniellee-ux]

Final Codex CLI review (codex exec review --base master, head 9019edc) verdict:

No actionable correctness defects were identified. The signing helper tests, diff checks, and macOS unit-test command completed successfully.

No changes were required from this review. GitHub Actions Tests / test also passed. A separate fresh-context reviewer is completing the final independent cross-check before merge.

[daniellee-ux]

Final independent merge-gate review: no actionable findings.

Cross-check scope included all three commits and related call sites, WhisperKit 0.18 / swift-transformers cache behavior, MainActor cancellation and latest-selection-wins paths, recording-session deferred switching, failure/reset and Retry/Dismiss behavior, Developer ID identity resolution, Bash 3.2/locale/pipefail behavior, and signing/notarization ordering.

Merge gates:

• Codex CLI review: clean
• Fresh-context transcription audit: clean
• Fresh-context packaging audit: clean
• GitHub Actions Tests / test: passed
• PR head verified at 9019edc
• PR state: CLEAN / MERGEABLE

Proceeding with a merge commit to preserve the intentionally separated commits.