docs(parts): examples, demo bundle and posts for the fastener library

[w1ne]

Announcement set for the fastener table shipped in #1019 (lib.part / lib.stack).

Contents

• Examples: examples/parts/fastener-lineup.kcad.ts (one envelope per family, M2–M6) and examples/parts/bolt-stack.kcad.ts (two bars, M5 bolt-mode stack; a 12 mm grip picks M5 × 18), plus examples/parts/verify.mts.
• Demo bundle docs/demos/standard-parts/: 23 s demo.mp4 composed from local kernelcad render --preset publish stills; retained frames, geometry-evidence.json (bounds, volumes, 15/28 pairs checked, zero positive-volume overlaps, source hashes), video-evidence.json, a render inspect bundle and README.
• Guide docs/guide/standard-parts-library.md: status: draft pending editorial approval, with 3 figures, 2 evaluated kcad blocks and 4 passing quote checks. Figures are staged in docs/guide/standard-parts-library/ per the draft flow.
• Posts: docs/posts/2026-10-17-parts-library-x.md (229 chars) and ...-linkedin.md (870 chars). Drafts only; nothing is posted.
• CHANGELOG: adds the entry feat(parts): one fastener table for lib.part and lib.stack #1019 shipped without.

Verification

• npx tsx examples/parts/verify.mts → {"ok":true,"screwLength":18,"overlaps":0}
• npx vitest run tests/integration/lib-parts.test.ts → 5 passed
• npx tsx scripts/typecheckExamples.ts examples/parts/bolt-stack.kcad.ts examples/parts/fastener-lineup.kcad.ts → OK
• npx tsx site/scripts/build-blog.ts --drafts → exit 0; draft preview built with video.mp4, model.glb and the three figures

Notes

• The guide stays unpublished while status: draft. To publish: move the three PNGs into site/blog/standard-parts-library/, repoint the figures, flip the status.
• Merging deploys docs/demos/** media on the next eligible develop push; the draft page itself is not built by a normal deploy.

[w1ne]

Cross-PR note: #1035 changes the countersunk head envelope from a cylinder to the seated ISO 10642 frustum (rim on the seat plane), so the sentence about "the flat-head cylinder" in this guide and the lineup render/video here are stale against it. After #1035 merges I will rebase this branch and refresh the wording, the lineup image and the demo video in one commit. Nothing else in this PR is affected.

[w1ne]

Refresh done in 1183d5c: merged develop (which now contains #1035) and updated for the seated countersunk envelope.

• Guide wording: the countersunk head is described as the seated ISO 10642 frustum, not a cylinder.
• Lineup render re-captured from the committed example; the screw now shows a real countersunk head.
• demo.mp4, its frames and video-evidence.json regenerated from the new render (23 s, hashes updated).
• CHANGELOG overlap resolved; blog checks pass (2 kcad blocks, 4 quotes).