fix(stringbuilder): guard capacity growth against Int overflow

[mizchi]

Summary

• Guard StringBuilder capacity growth against Int overflow.
• Add allocation-free whitebox coverage for normal growth and overflow boundaries.
• Add a focused native growth benchmark.

Root cause

StringBuilder::grow_if_necessary repeatedly doubled the backing capacity. Once the capacity exceeded 2^30, Int overflow could make the loop cycle through non-positive values and never reach the requested length. The new private helper falls back to the exact requested capacity after an overflowing double.

This is the StringBuilder counterpart to #3822, but it changes the separate UTF-16 builder implementation.

Validation

• git diff --check upstream/main...HEAD
• moon test -p builtin --target all --no-render passed in a local compatibility environment (wasm: 2887, wasm-gc: 2887, js: 2873, native: 2845).
• Native release benchmark, alternating baseline/head runs: no clear measurable regression within run-to-run variance.

The installed compiler currently rejects an unrelated pattern in builtin/iterator.mbt:789 on an unmodified upstream checkout; that is why the target test command needs the local compatibility environment above.

[Copilot]

Pull request overview

This PR hardens the non-JS StringBuilder (UTF-16 buffer-backed implementation) against Int overflow during capacity doubling, preventing potential non-terminating growth loops for very large required sizes.

Changes:

• Introduces a private stringbuilder_grow_capacity(current, required) helper that detects overflow during doubling and falls back to allocating exactly required.
• Updates StringBuilder::grow_if_necessary to use the new helper instead of an unguarded doubling loop.
• Adds allocation-free whitebox tests covering normal doubling behavior and overflow boundary cases, plus a focused benchmark for growth from the minimum capacity.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
builtin/stringbuilder_buffer.mbt	Adds overflow-safe capacity growth helper and wires it into grow_if_necessary.
builtin/stringbuilder_grow_wbtest.mbt	Adds whitebox tests validating normal doubling and overflow survival without allocating.
builtin/stringbuilder_bench_test.mbt	Adds a benchmark targeting growth behavior starting from minimal initial capacity.
builtin/moon.pkg	Excludes the new whitebox test from JS targets to match the non-JS implementation selection.

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