Add active-timeline opt-in and compile into app-created projects

[renezander030]

Superseded by #139, which contains all changes from this PR in the single v0.28.0 release.

Existing projects on unverified nested storage layouts can ignore edits to root mirrors. A newly generated project can also be rejected as coming from an unusual path. This adds two explicit alternatives for #50 and #52:

• --active-timeline follows the validated Timelines/project.json pointer. It rejects invalid, deleted, symlinked or conflicting selected documents, keeps root and active IDs, and preserves other timelines. Existing automatic selection stays unchanged. Library callers can use loadDraft(path, { activeTimeline: true }).
• compile --into <project> fills an empty project created in CapCut. It preserves each document's identity, app markers and unknown settings, copies content-correct media into project assets, and commits import registration with the timeline. It handles metadata files that also contain a timeline without duplicate writes. The shared store index is untouched.

Every root and active mirror must be empty. Changes during staging invalidate that check even with --force-write. Pointer and sidecar conflicts also remain mandatory; version and editor checks retain their existing behavior. Failed copies and partial commits clean up only newly owned assets. --check, --plan and --dry-run validate the destination without writing. Queue locks serialize compilation with edits through root and nested file aliases, and restore commits active mirrors together.

Validation: the required commit hook rebuilt the CLI and ran all 1,135 tests (1,134 passed; one platform-specific test skipped). Lint and 52 focused safety/queue tests pass. A fresh npm tarball passes the repository smoke suite and an additional actual-FFmpeg-media shell-compilation check covering intrinsic video/audio duration, source in-point, dimensions, registration links, public library options, unchanged store/pointer, and nonempty-project refusal. All 13 GitHub CI checks pass on a27bbed3889250a3a189dd21fd00c4ae2eea9315 (run), including Node 18/20/22, lint, Wasm, Linux/macOS/Windows smoke checks, and the Python client matrix.

CapCut desktop acceptance remains unverified for the new opt-in paths. Create an empty project in the app, quit CapCut, check the compile plan, compile into it, then open/save/close/reopen in the app to confirm persistence. These changes do not establish compatibility with every app version; #50 and #52 remain open pending that check.

This PR is based on #139 so its diff contains only the issue fixes. Merge it after the parent PR has been reviewed and merged. English/Chinese guidance and the command registry are updated; publication remains pending review.