perf: improve stats thresholds to reduce CI noise (#88158)
## Summary
Add per-metric significance thresholds that account for variance.
**Time metrics** (high variance from CI):
- Dev boot: <100ms AND <15%, OR <3%
- Build times: <500ms AND <5%, OR <2%
**Size metrics** (deterministic):
- node_modules: <10KB AND <1%, OR <0.01%
- Bundle sizes: <2KB AND <1%, OR <0.1%
## Problem
Current thresholds flag insignificant changes as regressions:
- 3.45 KB change on 457 MB node_modules (0.0008%) was flagged as a
regression
## Solution
Use per-metric thresholds with OR logic for very small percentage
changes:
- Time metrics need more generous thresholds due to CI variance
- Size metrics can be tighter since they're deterministic
## Test plan
- Verify that 3.45 KB on 457 MB (0%) is now marked as insignificant
- Verify that real regressions (>0.1% for sizes, >3% for times) are
still flagged
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Stack:
- #88157
- **→ This PR**
