fix(reporter): pad fractional part to two digits in number formatting

[paveg]

Summary

Fix a leading-zero display bug shared by four formatters in src/lib/reporter/format.mbt.
Each built its output by interpolating an unpadded 0–99 fraction,
so any fraction below 10 lost a digit:
80µs rendered as "0.8ms" instead of "0.08ms",
making percentile tables look non-monotonic (e.g. p90 "0.8ms" next to p95 "0.11ms").

Affected functions and examples (before → after):

• format_us_as_ms: 80µs "0.8ms" → "0.08ms"
• format_percentage: 33.05% "33.5%" → "33.05%"
• format_rps: 8.05 "8.5" → "8.05"
• format_time: 61.05s "61.5s" → "61.05s"

The fix pads the fraction to two digits, mirroring the pattern format_double already used.
Rounding/truncation behavior is unchanged.

Verification

• TDD: 7 new boundary tests written first and confirmed failing for the expected reason, then the fix applied
• moon test --target native 264/264, moon test (wasm-gc) 195/195
• All updated expectations are single zero-insertions into fractions; no assertion was weakened

🤖 Generated with Claude Code

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Latest commit: 7936063

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 63.79%. Comparing base (8fbb433) to head (7936063).
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #23      +/-   ##
==========================================
+ Coverage   63.59%   63.79%   +0.19%     
==========================================
  Files          16       16              
  Lines         739      743       +4     
==========================================
+ Hits          470      474       +4     
  Misses        269      269              

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