perf(bytes): skip redundant initialization in concatenation

[mizchi]

Summary

Avoid zero-initializing the concatenation result on non-JS targets before both inputs overwrite the full allocation. Allocate uninitialized storage, fill the two non-overlapping ranges with blits, then reinterpret the fully initialized buffer as Bytes. JS keeps the existing implementation because the required intrinsic is unavailable there.

Benchmark

Native release, 500,000 + 500,000 bytes:

Before	After
23.77 us	20.62 us

Validation

• moon test builtin --target all
• moon check
• moon info (no interface changes)

[Copilot]

Pull request overview

Optimizes Bytes concatenation on non-JS targets by avoiding redundant zero-initialization, while keeping the existing JS implementation for compatibility.

Changes:

• Refactors Bytes + Bytes to delegate to a target-specific bytes_add_impl and uses uninitialized allocation + blits on non-JS targets.
• Adds a regression test to ensure the concatenation result is fully initialized and correctly contains both inputs.
• Adds a benchmark covering large Bytes concatenation.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
builtin/bytes.mbt	Implements a non-JS fast path for Bytes::add using uninitialized allocation and two blits; keeps the JS path unchanged.
builtin/bytes_test.mbt	Adds a test validating Bytes::add initializes and populates the full result correctly.
builtin/bytes_add_bench_test.mbt	Introduces a benchmark for large Bytes concatenation.

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