ABI: implement exact internal length projection

[chrisbbreuer]

Parent: #134
Profile umbrella: #140
Home inventory: #163
Bun inventory: #164
Builds on: #222

Implement the pinned shared private-JSC export:

• JSC__JSValue__getLengthIfPropertyExistsInternal

Acceptance criteria:

• Return exact UTF-16 string length, Array logical length, every numeric TypedArray live length, and ArrayBuffer live byte length without observable property lookup.
• Return internal Map, Set, and WeakMap sizes while ignoring spoofed own/prototype length properties.
• Handle detached/out-of-bounds views and detached buffers with the pinned zero result.
• For other JSObjects, perform exactly one ordinary length lookup and ToNumber conversion, including accessors, proxies, inherited values, and abrupt completion.
• Return positive Infinity when a supported cell has no length, zero for non-cell/invalid/foreign inputs, and preserve the first pending VM exception.
• Focused tests cover strings with astral/lone-surrogate units, sparse arrays, all view widths, buffers, collection kinds, spoofing, missing length, conversion hooks, exceptions, and VM ownership.
• Extend the Home consumer fixture and publish exact Home/Bun inventory, ABI documentation, README, and parent counts.
• Batch the focused test, consumer fixture, both private audits, and C-API classification before publishing.
• Leave benchmark scores unchanged unless a benchmark path is rerun.

Completed evidence

Implemented on main in 70f91fa3; pinned inventories, ABI documentation, and README counts were published in 69a4206c. The type-directed projection now returns UTF-16 string length, Array logical length, numeric TypedArray live length, ArrayBuffer byte length, and internal Map/Set/WeakMap sizes without observable property reads. Other objects perform exactly one ordinary length lookup plus ToNumber; detached storage, missing values, unsupported cells, foreign inputs, proxies, accessors, conversion hooks, and abrupt completion follow the pinned result matrix.

The intentionally batched verification passed 15/15: focused tests passed 3/3 with zero leaks, the Home fixture linked and exercised 204/204 symbols, Home is 205/431 (226 pending), Bun core is 199/421 (222 pending), and the public audit reports 117 public, 19 extensions, and 209 private/support exports. Benchmark scores remain unchanged because no benchmark path was affected or rerun.

[chrisbbreuer]

Completed on main in 70f91fa3, with inventories and documentation in 69a4206c. All acceptance criteria are checked; the recorded batch passed 15/15 with zero leaks, fixture 204/204, Home 205/431, Bun 199/421, and 209 private/support exports.