docs: play README videos inline and document Lab owner review

[3dhlab]

The README currently opens repository file pages when visitors click its video posters. Replace those posters with GitHub video attachment players so both existing walkthroughs play inline. Keep the guides, timed text description, credits and permission notices; remove the README download links.

This documentation change fixes README playback and records the owner-authorized contribution policy; it needs no new release. The attached MP4s are byte-for-byte copies of the existing published files. The README now uses their stable attachment URLs. Both inline players load and play in a signed-out GitHub browser session, with durations of 64.5 and 40.7 seconds. The hosted player unmutes successfully. Required CI runs on the final documentation head. The owner authorized a review exception specifically for 3dhlab (user ID 324928685), in pull-request-only mode. A separate ruleset continues enforcing all four strict CI gates, a PR trail, resolved conversations and force-push/deletion protection for everyone. Unlock Digital and other contributors have no bypass or upstream merge authority. CONTRIBUTING.md now documents that Lab-authored PRs can be reviewed and merged by the Lab without external adoption. GitHub still does not count a formal author self-approval; the owner records its review decision before using the scoped exception.

Hosted walkthrough recording and editing: Unlock Digital. Permission to display participants in this edited video grants no reuse license for the video, its previews or depicted research content. See the repository media notices.

hosted-walkthrough.mp4

Local demo walkthrough: 3D Humanities Lab; generated content and first-party tutorial under Apache 2.0.

loci-demo-authoring.mp4

[3dhlab]

Owner review accepted for exact head d82608d. The two-file diff embeds the two existing, byte-identical walkthroughs in README.md and documents the owner-authorized Lab review policy in CONTRIBUTING.md. Guides, timed text, Unlock Digital recording/editing credit and hosted permission notices are preserved. Both GitHub players were independently verified while signed out; Craig also confirmed private-window visibility. The account-specific 3dhlab review exception is pull-request-only; all four strict CI gates, mandatory PRs, resolved conversations and history protections remain in a separate active ruleset with no bypass. Unlock Digital remains a fork contributor without upstream merge or exception authority. No new release is required or authorized for this documentation merge. All required checks on this exact head must pass before merging. GitHub records this as a comment review, because it cannot count a formal author self-approval.