ABI: implement exact private property-path traversal

[chrisbbreuer]

Goal

Implement the shared pinned Home/Bun JSC__JSValue__getIfPropertyExistsFromPath export with the exact observable behavior of Bun revision 4982b91e, then promote it from pending to implemented in both checked-in inventories.

Parent: #134
Profile umbrella: #140
Home profile: #163
Bun profile: #164

Acceptance criteria

• String paths follow the pinned UTF-16 parser exactly: [, ], and . are separators; empty, leading/trailing/consecutive-dot, separator-only, bracket, and Unicode cases preserve Bun/JSC behavior.
• Array paths accept only string and number elements, obtain length once with ToLength, perform indexed Get in order (including holes/prototypes/proxies), and use exact JavaScript number-to-string property keys.
• Every traversed value is boxed with ToObject; inherited properties, accessors, proxies, and primitive receivers behave observably like the pinned implementation.
• Present undefined remains distinct from an absent path; property lookup performs one observable Get per segment and does not introduce a separate has trap.
• Abrupt completion and pre-existing pending exceptions are first-wins and return the private empty sentinel; invalid or foreign boundary inputs fail deterministically.
• Focused runtime coverage and the combined private ABI consumer fixture link and execute the export.
• Home/Bun inventories, ABI documentation, parent trackers, and README counts are updated from the committed evidence.
• The relevant batched ABI audits and tests pass before closure.

Pinned reference

src/jsc/bindings/bindings.cpp at Bun revision 4982b91e, function JSC__JSValue__getIfPropertyExistsFromPath. Array traversal delegates to JSC forEachInArrayLike, i.e. ToLength followed by indexed Get for each entry.

Completion evidence

• Code: 2d169b1d — exact UTF-16 string grammar, JSC-style array-like traversal, one-Get segment lookup, VM ownership, and exception handling.
• Documentation: 860bd259 — README, ABI, API, and Home integration evidence/counts.
• Focused test: 3/3 passed, 0 skipped, 0 failed, 0 leaked.
• Batched ABI gate: Home 206/431 implemented (225 pending), Bun 200/421 implemented (221 pending), public C 117/117 plus 210 private/support exports.
• Consumer fixture: 205/205 symbols linked and the runtime matrix passed.
• Benchmark report remains the isolated 1,540-sample artifact at 36a617b3; this ABI-only slice did not regenerate performance scores.

[chrisbbreuer]

Completed on main in 2d169b1 with documentation/evidence in 860bd25. Focused traversal tests passed 3/3 with zero leaks; the batched Home/Bun/public audits and 205/205 consumer fixture are green.