Optimize performance for large-scale parallelism

[wangrunji0408]

This PR addresses performance bottlenecks in multithreaded scenarios by refactoring how Regex objects are handled.

The crate currently uses a globally shared Regex object, which internally maintains a cache pool. This implementation causes significant contention overhead when accessed by multiple threads simultaneously.

This PR changes Regex objects from global to thread-local storage. Each thread now compiles its own Regex upon first access and maintains an independent cache. We also add a new multithreaded benchmark to demonstrate the impact.

On a 11-core Macbook M3 Pro:

main:
multithreaded/multi_thread
                        time:   [118.90 µs 119.68 µs 120.63 µs]
                        thrpt:  [971.50 KiB/s 979.16 KiB/s 985.63 KiB/s]

this PR:
multithreaded/multi_thread
                        time:   [98.642 µs 99.572 µs 100.53 µs]
                        thrpt:  [1.1384 MiB/s 1.1493 MiB/s 1.1602 MiB/s]
                 change:
                        time:   [-18.014% -17.382% -16.666%] (p = 0.00 < 0.05)
                        thrpt:  [+20.000% +21.039% +21.972%]

On a 192-core x86_64 server:

main:
multithreaded/multi_thread
                        time:   [1.1241 ms 1.1523 ms 1.2042 ms]
                        thrpt:  [95.038 MiB/s 99.318 MiB/s 101.81 MiB/s]

this PR:
multithreaded/multi_thread
                        time:   [257.17 µs 258.42 µs 259.53 µs]
                        thrpt:  [440.96 MiB/s 442.85 MiB/s 445.01 MiB/s]
                 change:
                        time:   [-77.636% -77.141% -76.776%] (p = 0.00 < 0.05)
                        thrpt:  [+330.59% +337.47% +347.15%]

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[codecov]

Codecov Report

Attention: Patch coverage is 97.43590% with 2 lines in your changes missing coverage. Please review.

Project coverage is 84.25%. Comparing base (e6d6d9a) to head (41c215e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hmm.rs	95.23%	1 Missing ⚠️
src/lib.rs	98.24%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #122      +/-   ##
==========================================
- Coverage   85.76%   84.25%   -1.51%     
==========================================
  Files           9        9              
  Lines        2065     1912     -153     
==========================================
- Hits         1771     1611     -160     
- Misses        294      301       +7     

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[codspeed-hq]

CodSpeed Performance Report

Merging #122 will not alter performance

_{Comparing wangrunji0408:optimize-regex (41c215e) with main (e6d6d9a)}

Summary

✅ 10 untouched benchmarks
🆕 2 new benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
🆕	multi_thread	N/A	29.9 ms	N/A
🆕	single_thread	N/A	28.6 ms	N/A