fix(skills): accept durationless transition roots - #3683

What

Product-launch and PR-to-video transition injection now accepts a valid frame root without data-duration, inserts the outgoing transition target duration, and continues. A genuinely missing or mismatched composition root still fails hard.

Why

Composition root duration is optional and can be derived from its timeline. Both injectors checked duration before composition identity, so a durationless but correctly identified worker frame was skipped and then misreported as missing data-composition-id.

How

Both workflows now use the already-settled faceless-explainer ordering: identify the matching root first, replace its duration when present, or insert the target duration when absent. Non-root timed elements retain the existing tail-extension rules.

Test plan

• Product-launch inserts 3.5 seconds on a valid durationless outgoing root.
• PR-to-video inserts 3.5 seconds on the identical fixture.
• Both workflows retain the hard failure for a mismatched root.
• Shared transition regression passes (4/4).
• Existing PR-to-video frame-contract suite passes (9/9).
• Node syntax, oxlint, and oxfmt checks pass.
• Documentation updated (not applicable).