ABI: implement exact private record construction

[chrisbbreuer]

Goal

Implement the exact pinned Home/Bun private-JSC record/object construction boundary for:

• JSC__JSValue__putRecord
• JSC__JSValue__fromEntries

This is a child of #134, #140, #163, and #164.

Required semantics

JSC__JSValue__putRecord

• Decode the target in the selected VM and reject invalid/foreign targets deterministically.
• Decode the key and every value from all supported ZigString encodings without observable JavaScript coercion.
• Preserve the pinned cardinality rule: zero values creates an empty array, one value creates a scalar string, and multiple values create an ordered string array.
• Define a direct own data property with writable, enumerable, and configurable attributes, bypassing inherited setters.
• Replace an existing own property with the exact constructed value.
• Publish abrupt completion, allocation failure, and pre-existing pending exceptions through the shared first-wins exception boundary.

JSC__JSValue__fromEntries

• Construct an ordinary selected-realm object with Object.prototype, including the zero-entry case.
• Decode keys and values from all supported ZigString encodings without invoking JavaScript.
• Preserve insertion order, duplicate-key last-value behavior, and ECMAScript integer-index enumeration ordering.
• Match the pinned clone=false and clone=true ownership/lifetime behavior.
• Handle null pointers, count/capacity overflow, and allocation failure without partial observable results.
• Publish failures through the shared first-wins exception boundary.

Completion evidence

• Add focused behavioral tests for both exports, including Latin-1/UTF-8/UTF-16 strings, duplicates, numeric keys, inherited setters, cardinalities, realm/VM rejection, pending exceptions, and allocation cleanup.
• Add both symbols to the pinned Home/Bun inventories and the linkable private-value fixture.
• Keep public/private export classification at zero unclassified symbols.
• Run the focused test first, then one batched Home/Bun/public ABI audit.
• Update the README and compatibility documentation with exact counts and evidence.
• Close this issue only after the implementation and documentation commits are on origin/main.

[chrisbbreuer]

Completed on origin/main in implementation commit 8b564d0 and documentation commit eee6386. Focused record-construction coverage passed 3/3 with zero leaks. The batched gates pass at Home 214/431 (217 pending), Bun 208/421 (213 pending), 213/213 linked runtime symbols, 117/117 public declarations, 19 extensions, and 218 classified private/support exports. The committed JSC comparison remains 36a617b because this ABI-only slice does not change benchmark execution paths.