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-config/src/signed_policy/tests.rs

@@ -650,7 +650,8 @@ fn signed_cache_compromised_respects_signed_opt_out() {
     let dir = tempfile::tempdir().unwrap();
     let home = dir.path();
     let (kp, pubkey) = test_keypair();
-    let p = payload(); // fail_closed = false
+    // fail_closed = false
+    let p = payload();
     write_policy(home, &p);
     write_sidecar(home, &sign(&kp, &p)).unwrap();