ABI: return owned BunString values for private BigInt conversion

Parent

• #134
• #140
• #163
• #164

Scope

Implement the pinned shared JSC__JSBigInt__toString private ABI exactly enough for Bun/Home consumers:

• mirror the 64-bit 24-byte BunString tagged-union layout and exact tag values;
• return an owned, Latin-1 WTFStringImpl-tagged decimal result;
• provide compatible atomic ref/deref ownership for the returned string storage;
• preserve pending-exception and out-of-memory boundary behavior;
• cover signed, oversized, ownership, layout, and cross-realm cases in the focused ABI fixture;
• update both pinned inventories, ABI docs, roadmap progress, and README.

The pinned Bun implementation is src/jsc/bindings/JSBigIntBinding.cpp at 4982b91e; Home declares the same by-value ABI at 7ed99c02.

Acceptance

• Exact layout/tag/refcount contract is documented and compile-time asserted.
• JSC__JSBigInt__toString returns owned decimal text for every supported BigInt magnitude.
• Caller ref/deref behavior is safe and leak-free.
• Focused Home private ABI fixture and both pinned audits pass.
• README and parent issues carry current counts/evidence.

Evidence

Implemented in a216853e; documentation published in c944a3f5.

• zig build test-home-private-abi -Doptimize=Debug: 80/80 combined symbols linked and runtime matrix passed.
• zig build home-private-abi-audit bun-private-abi-audit: Home 82/431 implemented (349 pending); Bun 75/421 implemented (346 pending).
• Runtime coverage includes exact 24-byte/8-byte layout, tag values, StringImpl prefix/8-bit flag, signed and arbitrary-size decimal output, fresh refcount-one allocations, atomic retain/release, cold Rust destroy compatibility, same-VM sibling realms, and first-pending-exception preservation.

Completed in two pushed mainline chunks:

• a216853e — exact BunString/WTFStringImpl implementation, ownership helpers, inventories, and 80/80 focused runtime fixture.
• c944a3f5 — README and integration/API evidence with current profile counts.

Verification passed:

• zig build test-home-private-abi -Doptimize=Debug
• zig build home-private-abi-audit bun-private-abi-audit

Current audited status: Home 82/431 implemented (349 pending); Bun 75/421 implemented (346 pending). Parent issues #134, #140, #163, and #164 are updated.