docs(comments): rewrite comments across all crates to the guidelines

Sweep every first-party crate source (1956 .rs files) to the project comment
guidelines: delete redundant restatements, decorative banners, change
narration, and end-of-line comments; keep and tighten the crucial ones
(invariants, bug rationale, SAFETY blocks, ported-source attribution).

No functional code changed. Every edit is proven comment-only against the
prior tree by a comment-stripping lexer (string/char/raw-string aware) plus a
separate doctest-fence check. Where removing a comment made rustfmt or clippy
want to re-lay-out adjacent code, the minimal triggering comment is restored so
code tokens stay byte-identical.

Gates green: cargo fmt --all --check (0 diffs), cargo check and cargo clippy
--workspace --all-targets (0 warnings).

Adds scripts/check_codegen_comment_guidelines.py — the enforcement gate for
these guidelines (flags banners, end-of-line comments, change narration, and
commented-out code).

crates/codegen/kigi-test-support/src/counting_server.rs

@@ -1,6 +1,6 @@
-//! Minimal connection-counting HTTP/1.1 server for wire-level tests that need
-//! to assert TCP connection reuse (e.g. shared-client pooling): it counts
-//! accepted connections and records each request's header block.
+//! Minimal keep-alive HTTP/1.1 server for wire-level tests that assert TCP
+//! connection reuse (e.g. shared-client pooling): it counts accepted
+//! connections and records each request's header block.
 
 use std::sync::atomic::{AtomicUsize, Ordering};
 use std::sync::{Arc, Mutex};
@@ -8,8 +8,7 @@ use std::sync::{Arc, Mutex};
 use tokio::io::{AsyncReadExt, AsyncWriteExt};
 use tokio::net::TcpListener;
 
-/// Minimal keep-alive HTTP/1.1 server: counts accepted connections and
-/// records each request's header block.
+/// Returns the base URL, the accepted-connection count, and the header blocks.
 pub async fn spawn_counting_server() -> (String, Arc<AtomicUsize>, Arc<Mutex<Vec<String>>>) {
     let listener = TcpListener::bind("127.0.0.1:0").await.unwrap();
     let base_url = format!("http://{}/v1", listener.local_addr().unwrap());