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

crates/common/src/publisher.rs

@@ -6,10 +6,10 @@ use crate::backend::BackendConfig;
 use crate::http_util::{serve_static_with_etag, RequestInfo};
 
 use crate::constants::{COOKIE_SYNTHETIC_ID, HEADER_X_COMPRESS_HINT, HEADER_X_SYNTHETIC_ID};
-use crate::request_timer::RequestTimer;
 use crate::cookies::create_synthetic_cookie;
 use crate::error::TrustedServerError;
 use crate::integrations::IntegrationRegistry;
+use crate::request_timer::RequestTimer;
 use crate::rsc_flight::RscFlightUrlRewriter;
 use crate::settings::Settings;
 use crate::streaming_processor::{Compression, PipelineConfig, StreamProcessor, StreamingPipeline};