fix(media-use): preserve bundled SFX durations

[miguel-heygen]

Summary

• preserve exact bundled SFX durations when freezing media-use resolver records
• align every bundled SFX manifest duration with the frozen MP3 reported by ffprobe
• enforce full manifest-to-library coverage and exact duration integrity
• regress the real resolve boundary so a following audio clip can start at the frozen file endpoint

Root cause

The bundled manifest stored rounded durations, and the generic resolver rounded provider metadata again to tenths of a second. A 0.574688-second frozen effect was therefore recorded as 0.6 seconds, creating a false same-track overlap when the next clip began at the actual media boundary.

TDD evidence

The new tests were run against the previous implementation first:

• manifest integrity failed at chime.mp3: 2.5 !== 2.544
• bundled resolve boundary failed: 0.6 !== 0.574688

Verification

• bun run test:skills — 247 passed
• node --test skills/media-use/scripts/lib/bundled-sfx-provider.test.mjs — 2 passed
• node skills/media-use/scripts/resolve.test.mjs — 37 passed
• bun run lint:skills — 20 skill files clean
• bunx oxfmt --check on all changed media-use files
• git diff --check

Human review required; do not self-merge.