Fix retry bucket corruption on Julia 1.10

[quinnj]

Summary

• replace the atomic Set{String} retry depletion snapshot with a pointer-free atomic counter
• preserve the lock-free healthy path and the HTTP 2.6.6 six-field constructor
• add concurrent retry and GC regression coverage for Julia 1.10
• add Julia 1.10 to CI and guard Julia 1.11-only public API checks
• bump the package version to 2.6.7

Root cause

Retry bucket writers already hold bucket.lock. The crash is not caused by a missing writer lock. On Julia 1.10, publishing a GC-managed Set through the atomic field can corrupt the set's backing dictionary during concurrent requests and garbage collection. Tracking only the number of depleted partitions keeps the atomic fast path pointer-free. When the count is nonzero, replenishment takes the existing lock and checks the requested partition.

Validation

• full HTTP test suite on Julia 1.10.11
• full HTTP test suite on Julia 1.12.6, including 69 JuliaC trim checks
• exact Genie 6.0.4 reproduction on Julia 1.10.11 with one thread: 446 passed, 1 expected broken
• focused retry suites on Julia 1.10 and 1.12 after the version bump
• git diff --check

Fixes #1355

Co-authored by Codex

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.43%. Comparing base (eee3447) to head (3f8e33f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1356      +/-   ##
==========================================
+ Coverage   88.89%   89.43%   +0.53%     
==========================================
  Files          31       31              
  Lines       12134    12594     +460     
==========================================
+ Hits        10787    11263     +476     
+ Misses       1347     1331      -16     

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