Profiling, benchmarking instrumentation and optimization plan (#322)

* Add Server-Timing instrumentation and optimization plan

Introduce RequestTimer for per-request phase tracking (init, backend,
process, total) exposed via Server-Timing response headers. Add
benchmark tooling with --profile mode for collecting timing data.
Document phased optimization plan covering streaming architecture,
code-level fixes, and open design questions for team review.

Author: prk-Jr

.gitignore

@@ -26,3 +26,6 @@ src/*.html
 
 # SSL certificates
 *.pem
+
+/guest-profiles
+/benchmark-results/**