ABI: implement exact AggregateError construction and access

[chrisbbreuer]

Parent

• Roadmap: implement every README-declared missing runtime surface #134
• ABI: define and implement versioned Bun and Home private-JSC profiles #140
• ABI: implement the pinned Home private-JSC profile #163
• ABI: implement the pinned Bun private-JSC profile #164

Scope

Implement the three shared pinned AggregateError bridges:

• JSC__JSGlobalObject__createAggregateError from an encoded error slice and ZigString message.
• JSC__JSGlobalObject__createAggregateErrorWithArray from an existing exact JSArray, by-value BunString message, and optional cause.
• JSC__JSValue__getErrorsProperty as a direct own-property read that bypasses mutable prototypes/getters.

Acceptance

• Slice construction preserves order and exact value identity in a fresh selected-realm errors array.
• Existing-array construction preserves the exact array object and cause identity.
• Message decoding covers tagged ZigString and BunString representations, including lone surrogates.
• errors/message/cause descriptors and immutable AggregateError classification match pinned behavior.
• Same-VM siblings, foreign values, invalid arrays, failure atomicity, and first-exception preservation are covered.
• Focused runtime, both inventories, README/docs, and parent trackers are current.

Completion evidence

Implemented on main in 454b9d5e; README and ABI documentation published in ff808cd9.

• Slice construction preserves order and exact value identity in a fresh selected-realm errors array.
• Existing-array construction preserves the exact array and optional cause identities.
• Tagged ZigString and by-value BunString messages cover astral and lone-surrogate text.
• message/errors/cause descriptors are writable, non-enumerable, and configurable; AggregateError classification uses immutable metadata.
• Direct errors reads bypass prototypes; siblings, foreign inputs, invalid arrays, failure atomicity, and first exceptions are covered.
• zig build test-home-private-abi -Doptimize=Debug passes 103/103 symbols.
• Home audit: 105/431 implemented, 326 pending. Bun core audit: 98/421 implemented, 323 pending.

[chrisbbreuer]

Completed on main in 454b9d5 with documentation in ff808cd. Focused runtime: 103/103; Home: 105/431; Bun core: 98/421.