Boron Context v0.7.7: instrument external source sizes

[kforris]

Summary

• measure explicitly referenced local text files from file metadata only when they are inside confirmed registered project roots
• label directories as not applicable and expose specific unavailable reasons for remote, missing, unsupported, non-local, and out-of-root sources
• preserve historical records and avoid arbitrary URL fetching

Verification

• npm run check: 69 passed, 13 environment-skipped; held-out recall@5/MRR/source coverage 1.00; wrong-project retrieval/writeback 0
• isolated PostgreSQL migrations and integration: 13 passed
• Swift: 5 passed plus release build
• format, plugin cache digest, npm audit, package dry-run: passed

Release boundary

This is the single 0.7.7 productization milestone. It does not authorize public release, Marketplace submission, paid activity, or marketing.