perf(stringbuilder): reuse unshared reset buffer

[mizchi]

Summary

• Reuse StringBuilder storage on reset() unless a zero-copy to_string() result shares it.
• Store spare capacity as UninitializedArray[UInt16], initializing only 0..<len and avoiding redundant zero-fills before writes.
• Add whitebox coverage for grow/reset/string-conversion safety and large-buffer benchmarks.

Benchmark

Apple M5, MoonBit 0.1.20260824, native release. Three sequential runs per revision; lower is better.

Build size	Before (93044073)	After (48486332)	Improvement
1 MiB	28.06 µs	16.34 µs	41.8%
8 MiB	273.48 µs	152.75 µs	44.1%
32 MiB	1.30 ms	0.89 ms	31.4%

Command: moon bench --release --target native builtin/stringbuilder_large_bench_test.mbt

Validation

• moon test builtin --target all: wasm 2960, wasm-gc 2960, JS 2935, native 2919; all passed
• moon check
• moon info; no public interface 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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