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-pager-pty-harness/tests/empty_enter_send_now.rs

@@ -5,7 +5,8 @@
 //! ```
 
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
-#[ignore] // opt-in: real pager binary in a PTY (CI runs with --ignored)
+// opt-in: real pager binary in a PTY (CI runs with --ignored)
+#[ignore]
 async fn empty_enter_force_sends_top_queued() {
     kigi_pager_pty_harness::scenarios::empty_enter_send_now::assert_empty_enter_force_sends_top_queued()
         .await