Fix screen-reader action feedback

[baixiangcpp]

Summary

• prevent JSON Format/Minify from announcing success when parsing fails, cancel stale worker results on input/tree replacement, and route real Monaco shortcuts through the shared action toolbar
• make toolbar async state safe for concurrent actions and preserve newer announcements when older Promise actions settle
• remove duplicate toast and external-request announcements while preserving first-interaction visual feedback through a performance-safe dynamic toaster queue
• add regression coverage for invalid/success actions, Monaco propagation, stale workers, concurrent actions, Strict Mode toast replay, single-node external status, and first-copy smoke behavior

Real assistive-technology evidence

Validated the final candidate on Windows with NVDA 2026.1.1 and Chrome 150.0.7871.46:

• invalid Format and Minify: detailed JSON alert announced once; no contradictory completed announcement
• valid Format and Minify: success announced once
• real Monaco Ctrl+Enter / Ctrl+Shift+Enter: action ran, success announced once, and no newline was inserted
• JSON Copy: success announced once from Sonner, without mirror/toolbar duplication
• Pipeline sample/run: template, run log, and completion announcements each occurred once
• YouTube external-request Sample/confirmation: Permission and Ready messages each occurred once from a single text-node live source; no external request was made

Only the minimal target time windows will be posted to the issue. The full NVDA log is intentionally not uploaded because it contains unrelated desktop-session output.

Validation

• npm run lint (0 errors; one warning in generated coverage/block-navigation.js)
• npm test (319 files; 1733 passed, 3 skipped)
• npm run test:coverage -- --maxWorkers=4 (83.06% statements, 72.55% branches, 89.89% functions, 86.5% lines)
• npm run validate
• npm run build
• npm run test:e2e:smoke
• npm run test:e2e:pwa
• independent multi-pass diff review: no remaining actionable finding

Closure boundary

Refs #263. Keep the issue open: this desktop NVDA pass does not substitute for the required physical Android TalkBack or iOS VoiceOver evidence.

[baixiangcpp]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Keep it up!

Reviewed commit: adf4a4b66b

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