ABI: implement exact private property access and mutation

Parent

• #134
• #140
• #163
• #164

Scope

Implement the pinned Home/Bun private property-boundary imports with JavaScriptCore-compatible observable behavior:

• JSC__JSValue__createObject2
• JSC__JSValue__put
• JSC__JSValue__putToPropertyKey
• JSC__JSValue__deleteProperty
• JSC__JSValue__getIfPropertyExistsImpl
• JSC__JSValue__getPropertyValue
• JSC__JSValue__getOwn
• JSC__JSValue__getOwnByValue

Acceptance criteria

• createObject2 creates a selected-realm ordinary object and defines key 2 before key 1 with writable/enumerable/configurable data descriptors; duplicate keys retain key 2's insertion position but key 1's value.
• ZigString put performs a direct named own write, bypassing inherited setters, while property-key putToPropertyKey performs ToPropertyKey once, preserves symbols, handles indexes, and publishes coercion/write exceptions.
• deleteProperty uses ordinary [[Delete]], including proxy traps and configurability rules, and returns false for non-objects and exceptions.
• getPropertyValue performs ordinary prototype-aware [[Get]], including indexes, accessors, and proxies; absence returns the deleted sentinel and exceptions return empty.
• getIfPropertyExistsImpl implements Bun's non-index prototype-pollution-mitigated lookup: walk own properties/proxies but stop before the selected realm's %Object.prototype%; absence returns deleted and exceptions return empty.
• getOwn and getOwnByValue use GetOwnProperty, invoke own accessors/proxy traps, support BunString/Empty and exact property-key coercion/index paths, and return empty for absence or exception.
• Focused ABI coverage exercises duplicate-key order, direct-write bypass, numeric and symbol keys, inherited and own accessors, proxy traps, non-configurable delete, prototype cutoff, non-object sentinels, throwing coercion/accessors/traps, cross-realm values, and pending-exception preservation.
• Home/Bun implementation inventories, ABI documentation, README progress, and parent trackers are updated from checked-in evidence.

Implemented and published directly on main.

Implementation: 28eb0306 (feat(abi): implement private property boundary (#203)) Documentation: 38629616 (docs(abi): publish property boundary evidence (#203))

Delivered:

• all eight pinned exports: createObject2, direct ZigString put, single-coercion putToPropertyKey, ordinary deleteProperty, normal and Object.prototype-cutoff reads, plus BunString/value-key own reads;
• a found-aware ordinary [[Get]] path that distinguishes absent from present-undefined without adding a Proxy has trap;
• exact key-2-first duplicate insertion semantics, selected-realm prototypes, numeric/Symbol/property coercion, Latin-1 names, accessor/proxy behavior, empty/deleted exception sentinels, sibling identity, foreign rejection, and first-exception preservation;
• refreshed Home/Bun inventories and README/API/Home integration evidence.

Validation:

• zig build test-home-private-abi -Doptimize=Debug — green, 111/111 symbols linked and runtime matrix passed.
• zig build home-private-abi-audit bun-private-abi-audit — green.
• Home: 113/431 implemented, 318 pending.
• Bun core: 106/421 implemented, 315 pending.
• Public/extension accounting remains 117/19.

README benchmark scores were preserved because this was an ABI/correctness slice and no benchmark inputs changed.