Conformance matrix execution and benchmark CI

[FumingPower3925]

Summary

Execute the full conformance suite (#13) across the complete engine × protocol matrix with full API integration, and establish benchmark regression tracking.

Conformance Matrix

7 engine configurations × 3 protocols = 21 combinations:

Engine Config	HTTP/1.1	H2C	Auto
io_uring Base	✓	✓	✓
io_uring Mid	✓	✓	✓
io_uring High	✓	✓	✓
io_uring Optional	✓	✓	✓
epoll	✓	✓	✓
StdEngine	✓	✓	✓
Adaptive	✓	✓	✓

CI Benchmarks

Run all 5 benchmarks from #13 against every engine configuration:

• BenchmarkPlaintext
• BenchmarkJSON
• BenchmarkPathParam
• BenchmarkBodyRead
• BenchmarkRealisticAPI

Regression Detection

• Store benchmark baselines as CI artifacts
• Compare against previous baseline on each PR
• Fail CI if any benchmark regresses >10% in ns/op or allocs/op
• SDD 14.5 performance regression benchmarks tracked as CI metrics

Deliverables

• Matrix test runner executing all 21 combinations
• Benchmark baseline storage and comparison
• CI job with regression gate (>10% = failure)
• Clear regression report in CI output

Tests

• All 21 matrix combinations pass
• Benchmark baselines stored correctly
• Regression detection triggers on >10% degradation
• CI output clearly identifies which benchmark regressed

SDD Reference

Sections 14.1, 14.5

Dependencies

Depends on #13, #19, #20

[FumingPower3925]

CI workflow exists (.github/workflows/ci.yml) with lint, test, conformance, spec, integration, vulncheck, and bench jobs.