Improve taint analyzer performance with shared SSA cache, parallel analyzer execution, and CI regression guard

[ccojocar]

• Added a shared per-package SSA analysis cache with lazy, concurrency-safe call graph reuse across analyzers.
• Updated taint analyzers to consume the shared cache instead of recomputing expensive artifacts per rule run.
• Parallelized analyzer execution at package level while preserving deterministic issue aggregation.
• Added a package-level taint benchmark to measure real end-to-end taint analyzer pass performance.Introduced a CI benchmark regression guard with configurable thresholds for ns/op, B/op, and allocs/op.
• Documented the performance guard workflow, local run command, and baseline update process in the README.

[codecov]

Codecov Report

❌ Patch coverage is 80.35714% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.08%. Comparing base (b940702) to head (cae924a).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
analyzer.go	87.50%	3 Missing and 2 partials ⚠️
internal/ssautil/package_analysis_cache.go	60.00%	2 Missing and 2 partials ⚠️
taint/taint.go	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1530      +/-   ##
==========================================
+ Coverage   76.04%   80.08%   +4.03%     
==========================================
  Files         100      101       +1     
  Lines        8688     9175     +487     
==========================================
+ Hits         6607     7348     +741     
+ Misses       1698     1382     -316     
- Partials      383      445      +62     

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