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-mcp/tests/repro_sse_flood.rs

@@ -150,8 +150,8 @@ async fn throttled_client_bounds_the_flood() {
     );
     let client = ().serve(transport).await.expect("handshake against fake server should succeed");
 
-    // Checkpoint: backoff must engage early (instant, instant, 0.5s => 3-4
-    // GETs by 1.2s; an unthrottled client would be in the hundreds).
+    // The schedule is instant, instant, 0.5s, so 1.2s allows 3-4 GETs; an
+    // unthrottled client would be in the hundreds by now.
     tokio::time::sleep(Duration::from_millis(1200)).await;
     let early = gets.load(Ordering::Relaxed);
     assert!(