[Feature] Start direct Replay HQ export from Timeline preparation

[chdenat]

Context

After inspecting a linked Replay through the read-only Timeline preparation surface, the user must be able to start the final HQ video without creating an intermediate Draft recording.

The HQ exporter must continue to use the canonical fixed-frame Replay timeline and the isolated render target already defined by the replay architecture.

Requested behavior

Add a direct HQ export action from linked Timeline preparation.

The export must:

• consume the same logical timeline and Replay definition shown in the Timeline preview;
• render deterministic fixed frames through the existing HQ exporter;
• preserve Start, Replay, Stop, trace, camera, and Replay widget visibility behavior;
• keep the interactive preview independent while HQ rendering is active;
• exclude the transient Timeline widget from the captured board and output;
• expose normal preparation, progress, cancellation, completion, and failure states;
• clean up the preview and HQ resources on every terminal path.

Acceptance criteria

• A linked preparation session can start HQ export without creating a Draft blob first.
• HQ frame timestamps and total duration match the Timeline projection.
• HQ output contains the expected market, trace, camera, and Replay widget states.
• The interactive preview remains usable and is not used as the HQ camera/render authority.
• The Timeline widget is never included in the encoded output.
• Cancellation, failure, and completion release the HQ render target and transient Timeline state.
• Existing non-linked HQ export behavior remains unchanged.
• Integration tests cover direct start, frame/timeline parity, capture exclusion, cancellation, and cleanup.

Notes or questions

The user-facing action label can follow the existing product terminology, such as Create video or Export HQ, once the preparation UI is implemented.

Proposed solution and implementation plan

1. Expose a direct linked-preparation command that builds the existing HQ export plan.
2. Reuse the normalized Timeline projection and fixed-frame scheduler rather than reading UI pixels or wall-clock time.
3. Route export frames through the existing isolated HQ render host and composition pipeline.
4. Keep the Timeline as an external transient control surface.
5. Reuse the existing monitor, cancellation, final preview, and cleanup lifecycle.
6. Add end-to-end/integration tests for deterministic timestamps, output composition, and terminal cleanup.