refactor(string): iterate code units directly

[Yu-zh]

Summary

• Replace simple index-based string and string-view code-unit loops with direct for .. in code_units() iteration.
• Keep index variables only where they are still needed for output positions or slicing.
• Leave allocation-sensitive hashing and low-level string blit paths indexed.

Test Plan

• moon fmt
• moon check
• moon test (6521/6521 passed)
• moon info

[coveralls]

Coverage Report for CI Build 4648

Coverage remained the same at 94.247%

Details

• Coverage remained the same as the base build.
• Patch coverage: 1 uncovered change across 1 file (24 of 25 lines covered, 96.0%).
• 1 coverage regression across 1 file.

Uncovered Changes

File	Changed	Covered	%
encoding/base64/decode.mbt	6	5	83.33%
Total (8 files)	25	24	96.0%

Coverage Regressions

1 previously-covered line in 1 file lost coverage.

File	Lines Losing Coverage	Coverage
encoding/base64/decode.mbt	1	85.0%

---

Coverage Stats

Relevant Lines:	15869
Covered Lines:	14956
Line Coverage:	94.25%
Coverage Strength:	198649.58 hits per line

---

💛 - Coveralls

[Yu-zh]

@codex review

[chatgpt-codex-connector]

💡 Codex Review

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Reviewed commit: 8d3e8d4dbd

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