Mobile playback: add 10-second touch seek controls #111

Part of #27 and the native Android half of #8.

Outcome

A person watching video on Android can seek without dragging the timeline: double-tap the left or right playback region, or use visible accessible controls, and see immediate accumulated feedback.

Scope

• Double-tap left/right seeks by 10 seconds.
• Repeated taps accumulate predictably and show the pending total.
• Clamp the target at the start and duration bounds.
• Preserve single-tap control toggling, captions, and system navigation behavior.
• Provide visible TalkBack-accessible back/forward controls with the same outcome.
• Disable seeking for live or otherwise non-seekable media.
• Keep gesture state and player effects explicit and lifecycle-safe.

Acceptance criteria

• Focused reducer/interaction tests cover accumulation, clamping, conflicts, accessibility, and non-seekable media.
• Canonical Gradle verification and both production-debug assembles pass.
• API 37 proof passes.
• A screen recording demonstrates both gestures, accumulated feedback, boundary clamping, and accessible controls: https://attach.uinaf.dev/p/LNKhR7uc6RkmvTqT91AJrA and https://attach.uinaf.dev/p/zDFSDvpX6PF_KqSj20rLiw on client 9677. TalkBack activation cannot be recorded on the emulator (injected input bypasses explore-then-double-tap); TalkBack focus is proven and a device recording is tracked under Physical device proof set: Android TV, Fire TV, phone, tablet inventory #51.

Dependencies

Playback foundation #88/#89 is merged and authenticated playback now works. PR #112 remains a draft: the gesture/clamping recordings exist, but TalkBack seek activation still needs a recording. TalkBack focus on both buttons is verified; emulator input timing and keyboard routes have not established activation. Two current code-review findings are also under validation in the PR.