fix: announce simulation speed changes

[mittalsonal]

Related Issue

Closes #2078

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Change Summary

• Added a polite, atomic role="status" live region to the Time Controls panel.
• Routed simulation speed button clicks through a handler that keeps current speed behavior and announces the selected multiplier, e.g. Simulation speed set to 10x.
• Added regression coverage for the live-region announcement and refreshed the RightSidebar store mock so current controls render correctly.
• Tightened touched-file lint typing in RightSidebar.tsx without changing unrelated app behavior.

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Suggested Area

Accessibility

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Core Files Changed

• src/components/RightSidebar.tsx
• src/__tests__/RightSidebar.test.tsx

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Verification

• Screen-reader-only announcement; no visible layout change.
• npm test -- --run src/__tests__/RightSidebar.test.tsx - passed, 3 tests.
• npm run typecheck - passed.
• npx eslint src/components/RightSidebar.tsx src/__tests__/RightSidebar.test.tsx --format json - passed, clean result for touched files.
• npm run build - passed.
• git diff --check -- src/components/RightSidebar.tsx src/__tests__/RightSidebar.test.tsx - passed.
• npm run lint -- --format json - repo-wide run still reports pre-existing findings outside these touched files; touched files are clean.

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AI Assistance Declaration

Did you use an AI tool to write or assist with this code OR Pull Request?

• Yes

• No

• Which AI Model did you use?: GPT-5 Codex

• Which Platform/Tool?: Codex

• What exactly did the AI do?: Assisted with repository inspection, the live-region implementation, test updates, and local verification.

• What exactly did YOU do?: Reviewed the assigned task, kept the change scoped, verified account and repository safety, and validated the implementation locally.

• What is the advantage of using this AI approach here?: Faster accessibility-focused implementation with explicit regression coverage and verification.

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Reviewer Notes

• This is intentionally screen-reader-only; there should be no visible layout change.
• The live region starts empty and announces only after a user chooses a speed multiplier.
• Existing local docs/ARCHITECTURE.md changes were not included in this PR.

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Local Verification Pledge

• I tested these changes in my own local branch.
• I verified this code compiles into a standalone build and does not break existing production behavior.

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