src/h264.ts and src/video-fanout.ts: split Annex-B access units and fan them out per viewer, skipping to the next keyframe - modules and tests only #257

From #70, piece 1 of 4. Pull #207 was closed so the work can land in smaller pulls. Take the code from pull #207's branch smith/70-real-video-hardware-h-264-from-the-pi-ov; three reviews accepted its design, caps, security and fallback.

Done:

• [ ] Software/so101/src/h264.ts (the bounded Annex-B splitter, without codecOf, which nothing calls) and src/video-fanout.ts (a per-viewer ladder that drops to the next keyframe rather than queueing, with control messages capped at 200 bytes), each with its test.
• [ ] Nothing imports them yet, and no other file changes.
• [ ] bun x tsc --noEmit and bun --cwd Software/so101 test pass.