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-shell/tests/test_global_extra_headers_e2e.rs

@@ -23,7 +23,8 @@ use kigi_test_support::*;
 /// Every global `[models]` default is accepted, and the wire-observable
 /// `extra_headers` reaches the inference request with no per-model block in play.
 #[tokio::test]
-#[ignore] // requires pre-built binary; run with --ignored
+// requires pre-built binary; run with --ignored
+#[ignore]
 async fn global_models_config_reaches_inference_request() {
     let server = MockInferenceServer::start()
         .await