Perf: hoist plain-object prototype lookup from allocation batches

Parent: #62

Evidence

Current main at f4602d86 still trails system JSC in the fair direct object_churn row. A fresh five-sample same-session check measured medians of about 197.000 ms for zig-js and 117.836 ms for JSC, with checksum 1030552128 in every sample (about a 1.67x remaining elapsed gap).

A fresh symbolized ReleaseFast profile shows the guarded fixed-shape allocation loop repeatedly entering Interpreter.newObject; a visible portion of that path is objectProto, which walks to the realm root and locks its binding table even though %Object.prototype% is an immutable intrinsic identity already cached by the realm. The quick batch validates one root shape and can resolve that intrinsic once before allocating its checkpoint-bounded run.

Scope

• expose a narrow intrinsic-prototype resolver for VM allocation tiers;
• resolve %Object.prototype% once per guarded allocation batch rather than once per fresh object;
• allocate/default-initialize every object through the normal GC seam and assign the same intrinsic prototype;
• preserve realm identity, bootstrap/null-prototype behavior, OOM recovery/state publication, tracing, checkpoints, and all existing quick-loop guards/fallbacks.

Acceptance criteria

• focused tests prove quick and generic execution retain identical values and exact VM step counts across realms;
• OOM and fallback behavior remain covered;
• exact-parent alternating object_churn A/B shows a repeatable win with checksum 1030552128;
• retain only if the signal justifies the added API seam;
• batched full validation and commits reference this issue.

Implemented and pushed in db131e0.

Exact-parent alternating A/B on object_churn (100 jobs, 3 samples/process):

• Batch 1: candidate won 6/7 pair medians; aggregate median 198.414 ms vs 201.122 ms (-1.35%).
• Reversed-order confirmation: candidate won 5/7 pair medians; aggregate median 202.508 ms vs 214.441 ms (-5.56%; noisier batch).
• All 84 samples produced checksum 1030552128.
• The quickened path now has a regression assertion for intrinsic Object.prototype identity after replacing the global Object binding.
• Validation: 760/760 unit tests, zero leaks; 7/7 comparison-harness tests.

The change resolves the realm intrinsic once per checkpoint-bounded allocation batch and preserves allocation failure publication and exact step accounting.