Make kaocha.jit/jit thread-safe

[frenchy64]

Ensures we don't race with other requiring threads.

Note this is now safer than requiring-resolve (as of Clojure 1.12), which has a race condition making it prone to returning a partially initialized var.

[alysbrooks]

The warning isn't a big deal either way, but I did look into the popularity of older Clojure versions, and 1.10 was already being phased out in 2022: https://clojure.org/news/2024/12/02/state-of-clojure-2024.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.15%. Comparing base (15e553d) to head (a5de958).
⚠️ Report is 24 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #453      +/-   ##
==========================================
- Coverage   69.17%   69.15%   -0.02%     
==========================================
  Files          52       52              
  Lines        2819     2824       +5     
  Branches      187      189       +2     
==========================================
+ Hits         1950     1953       +3     
- Misses        689      690       +1     
- Partials      180      181       +1     

Flag	Coverage Δ
unit	69.15% <100.00%> (-0.02%)	⬇️

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