[reilly1993]

Problem

The replay back and forward buttons used timestamps as their cursor. When several actions shared a timestamp, a single press could skip multiple moves or appear not to work.

Solution

• Track an explicit action index alongside replay time.
• Make manual back and forward controls move exactly one action and pause autoplay.
• Keep slider seeking and autoplay time-based.
• Cancel pending animation frames before manual stepping and restart cleanly after replay completion.
• Add accessible labels and focused coverage for shared timestamps, seeking, autoplay and restart behavior.

Verification

• 157 Bun tests pass.
• TypeScript typecheck passes.
• Biome passes across src.
• Production Vite/PWA build passes.
• Manually verified in the local replay UI.