refactor: Preallocate UTF-8 encoding output

[peter-jerry-ye]

Summary

• replace @encoding/utf8.encode's buffer-based implementation with a two-pass encoder that sizes the UTF-8 output first and writes directly into one FixedArray[Byte]
• validate surrogate structure during encoding so malformed UTF-16 input aborts instead of being silently encoded
• add regression tests for valid encoding behavior and malformed surrogate panic cases

Why

The previous implementation encoded through Buffer::write_string_utf8, which iterated by code point and paid repeated grow checks on the hot path. This change keeps the work at the UTF-16 code-unit level, does one preallocation, and then writes bytes directly.

Impact

• improves UTF-16 -> UTF-8 encoding throughput
• preserves BOM behavior and existing output for valid inputs
• now rejects invalid surrogate pairs explicitly

Validation

• moon test encoding/utf8/encode_test.mbt --target all
• moon check encoding/utf8 --target all
• moon fmt
• moon info encoding/utf8
• moon bench -p tmp/utf8_encode_bench --target native --release
• moon bench -p tmp/utf8_encode_bench --target js --release
• moon bench -p tmp/utf8_encode_bench --target wasm --release
• moon bench -p tmp/utf8_encode_bench --target wasm-gc --release

Notes

The temporary tmp/utf8_encode_bench package used for measurement was removed before commit.

[coveralls]

Coverage Report for CI Build 3833

Coverage decreased (-0.02%) to 94.892%

Details

• Coverage decreased (-0.02%) from the base build.
• Patch coverage: 5 uncovered changes across 1 file (36 of 41 lines covered, 87.8%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
encoding/utf8/encode.mbt	41	36	87.8%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	15504
Covered Lines:	14712
Line Coverage:	94.89%
Coverage Strength:	220621.5 hits per line

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💛 - Coveralls

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional findings.