perf(stringbuilder): reuse unshared reset buffer

[mizchi]

Summary

• Reuse a StringBuilder backing buffer on reset() when it has not been exposed by the zero-copy to_string() path.
• Track the exceptional full-buffer case with a private buffer_is_shared flag and detach only then.
• Add a non-JS whitebox regression test that verifies reuse while retaining the snapshot contract.

Why

The native/wasm StringBuilder::reset implementation always allocated and zero-filled a new FixedArray[UInt16], even when to_string() had already copied the contents. This defeated the intended spare-capacity reuse.

A full buffer may be turned into a String without copying, so simply setting len = 0 would mutate a string previously returned by to_string(). The new flag preserves that case by allocating a fresh buffer only when necessary.

Benchmark

Existing benchmark: StringBuilder::reset reuse spare capacity n=4096. Lower is better.

Target	Before	After	Improvement
native	75.06 µs	52.32 µs	30%
wasm-gc	95.90 µs	81.63 µs	15%

Validation

• moon test builtin --target all --quiet
• moon check builtin --target all
• moon info builtin
• moon fmt builtin/stringbuilder_buffer.mbt builtin/stringbuilder_wbtest.mbt

No public API changes.

[Copilot]

Pull request overview

This PR optimizes the native/wasm builtin StringBuilder implementation by allowing reset() to reuse the existing UTF-16 backing buffer when it’s safe (i.e., when the buffer hasn’t been exposed via the zero-copy to_string() fast path), reducing allocations and zero-fills while preserving correctness.

Changes:

• Add an internal buffer_is_shared flag to track when to_string() returns a zero-copy string backed by the builder’s buffer.
• Update reset() to reuse the existing buffer unless it has been shared, in which case it detaches by allocating a fresh buffer.
• Add a non-JS whitebox regression test validating that reset() reuses an unshared buffer.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
builtin/stringbuilder_buffer.mbt	Track zero-copy sharing in to_string() and reuse backing storage in reset() when safe.
builtin/stringbuilder_wbtest.mbt	Add a non-JS whitebox test that asserts buffer reuse behavior across reset().

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