Optimize String code unit search with V128

[Yu-zh]

Summary

• add builtin V128 helpers for loading and comparing eight UTF-16 code units
• optimize String::contains_code_unit and StringView::contains_code_unit with eight-wide scans on native and wasm targets
• retain the scalar implementation for other targets and for short/tail ranges
• add SIMD block, view-boundary, surrogate-code-unit, and benchmark coverage

Why

Code-unit searches previously inspected one UTF-16 code unit at a time. The new intrinsic allows native and wasm backends to scan eight code units per iteration while preserving the existing behavior and public API.

Impact

Long missing or late-match searches are substantially faster on SIMD-enabled targets. String views continue to honor their exact backing-string bounds, and non-native/non-wasm targets keep the scalar path. Generated package interfaces are unchanged.

Benchmark

Isolated native ARM64 release runs comparing the scalar baseline with this implementation (lower is better):

Case	Scalar baseline	V128	Speedup
String, absent, n=4	7.84 ns	6.50 ns	1.21×
String, absent, n=8	8.81 ns	6.49 ns	1.36×
String, present at 0, n=8	5.77 ns	5.69 ns	1.01×
String, absent, n=256	82.26 ns	16.10 ns	5.11×
offset StringView, absent, n=256	86.38 ns	17.47 ns	4.94×

The SIMD path improves full scans substantially while keeping the immediate-match case effectively unchanged.

Validation

• moon fmt
• moon info (no .mbti changes)
• moon check --target all -p builtin
• moon test --target all -p builtin
  • wasm: 2,886 passed
  • wasm-gc: 2,886 passed
  • JavaScript: 2,874 passed
  • native: 2,844 passed
• moon test: 6,716 passed

[coveralls]

Coverage Report for CI Build 5169

Coverage decreased (-0.1%) to 91.111%

Details

• Coverage decreased (-0.1%) from the base build.
• Patch coverage: 24 uncovered changes across 2 files (5 of 29 lines covered, 17.24%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
builtin/simd.mbt	17	0	0.0%
builtin/string_methods.mbt	12	5	41.67%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	17291
Covered Lines:	15754
Line Coverage:	91.11%
Coverage Strength:	190188.93 hits per line

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💛 - Coveralls