feat: Sprint 1 Phase 3 - core stability, headers, and docs

[fr3on]

Description

Implemented custom header forwarding, configurable buffers, named constants, and full deployment documentation. Final phase of Sprint 1.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• PHPStan reports no errors

[github-actions]

🚀 Quill Performance Metrics

Environment: Ubuntu · PHP 8.3 · quill-core 0.0.2 · 2 workers Quill Core

Quill Binary Benchmark
  > Server     : Quill Core
  > Tool       : wrk
  > Duration   : 5s
  > Connections: 50
  > Workers    : 2
  > Starting Quill server...
  > Server is ready. Running benchmarks...
    Running 5s test @ http://127.0.0.1:8080/hello
      2 threads and 50 connections
      Thread Stats   Avg      Stdev     Max   +/- Stdev
        Latency     0.96ms  640.80us   6.77ms   71.54%
        Req/Sec    25.42k   682.32    28.11k    76.00%
      253202 requests in 5.01s, 39.12MB read
    Requests/sec:  50566.46
    Transfer/sec:      7.81MB

  > Benchmarks completed.