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-render/src/util.rs

@@ -262,12 +262,12 @@ mod tests {
     fn time_ago_days() {
         assert_eq!(format_time_ago(Duration::from_secs(86400)), "1d");
         assert_eq!(format_time_ago(Duration::from_secs(172800)), "2d");
-        assert_eq!(format_time_ago(Duration::from_secs(2_592_000 - 1)), "29d"); // just under 30d
+        assert_eq!(format_time_ago(Duration::from_secs(2_592_000 - 1)), "29d");
     }
 
     #[test]
     fn time_ago_months() {
-        assert_eq!(format_time_ago(Duration::from_secs(2_592_000)), "1mo"); // 30d
+        assert_eq!(format_time_ago(Duration::from_secs(2_592_000)), "1mo");
         assert_eq!(format_time_ago(Duration::from_secs(5_184_000)), "2mo");
         // 359d is still 11mo (359/30=11); 360d would be 12mo.
         assert_eq!(format_time_ago(Duration::from_secs(359 * 86400)), "11mo");
@@ -275,7 +275,7 @@ mod tests {
 
     #[test]
     fn time_ago_years() {
-        assert_eq!(format_time_ago(Duration::from_secs(31_536_000)), "1y"); // 365d
+        assert_eq!(format_time_ago(Duration::from_secs(31_536_000)), "1y");
         assert_eq!(format_time_ago(Duration::from_secs(63_072_000)), "2y");
     }