fix: prevent regexp cache charsCurrent inflation on duplicate Put

[cubic-dev-ai]

Summary

• Fix regexpCache.Put to only increment charsCurrent when the key is new, preventing size counter inflation from concurrent duplicate inserts
• Add test cases covering duplicate-key Put to verify correct accounting

Problem

CompileRegexp performs a non-atomic Get/compile/Put sequence. When multiple goroutines miss on Get for the same regexp concurrently, each calls Put, which blindly incremented charsCurrent without checking if the key already existed. This inflated the size counter while len(rc.m) stayed at 1, causing premature cache eviction and unnecessary regexp recompiles.

Fix

Added an existence check before incrementing charsCurrent:

if _, exists := rc.m[regexp]; !exists {
    rc.charsCurrent += len(regexp)
}
rc.m[regexp] = rcv

Test plan

• Extended TestRegexpCache with duplicate-key inputs
• Added TestRegexpCacheDuplicatePut verifying triple-Put only counts chars once
• All tests pass with -race detector
• go vet clean

🤖 Generated with Claude Code

---

Summary by cubic

Fix regexpCache.Put to count chars only for new keys and add a fast path for existing entries. Prevents counter inflation under concurrency and premature evictions; tests cover duplicates.

^{Written for commit 958928e. Summary will update on new commits.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.81%. Comparing base (8ba28d7) to head (958928e).
⚠️ Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #68      +/-   ##
==========================================
+ Coverage   89.67%   90.81%   +1.14%     
==========================================
  Files          11       11              
  Lines        2984     3257     +273     
==========================================
+ Hits         2676     2958     +282     
+ Misses        210      204       -6     
+ Partials       98       95       -3     

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[cubic-dev-ai]

No issues found across 2 files

[f41gh7]

CompileRegexp performs a non-atomic Get/compile/Put sequence. When multiple goroutines miss on Get for the same regexp concurrently, each calls Put, which blindly incremented charsCurrent without checking if the key already existed. This inflated the size counter while len(rc.m) stayed at 1, causing premature cache eviction and unnecessary regexp recompiles.

This commit adds a check, if parsed regex was already added by concurrent goroutine.