Bun ABI: generate the pinned core-JSC declaration inventory

Parent: #164 Depends on: #166

Establish Bun’s exact core src/jsc denominator separately from bundled C libraries and wider runtime bindings.

Acceptance:

• Pin Bun revision 4982b91 and every src/jsc file containing a legacy/private extern fn declaration.
• Record every unique symbol, normalized signature/digest, source location/hash, classification, status, and calling convention.
• Classify public-C overlap, platform imports, and private-JSC entries with zero duplicates/unclassified entries.
• Publish an exact Home-vs-Bun core source/symbol diff so neither profile is inferred from the other.
• Add revision/source/signature/status drift gates and keep broader Bun runtime/generated bindings explicitly out of this core slice.

Completed in d97a0bc and documented in ede1fe3.

The pinned Bun core audit reports 437 unique symbols from 54 src/jsc files: 422 private (4 implemented / 418 pending) and 15 public overlaps, with zero duplicates/unclassified. Every declaration has a signature/digest, source/line/hash, calling convention, classification, and status. The exact Home comparison is 434 shared names, 3 Bun-only, 14 Home-only, and 28 changed signatures. Wider runtime/generated/bundled-library surfaces remain explicitly outside this core slice.