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-fsnotify/src/state.rs

@@ -226,7 +226,7 @@ mod tests {
     }
 
     /// Settle expiry emits exactly one Completed comparing the first pick's
-    /// pre-op HEAD against the final HEAD (head_changed spans the merged op).
+    /// pre-op HEAD against the final HEAD (`head_changed` spans the merged op).
     #[test]
     fn settling_expiry_emits_completed_spanning_merged_op() {
         let now = Instant::now();
@@ -300,7 +300,7 @@ mod tests {
     }
 
     /// Regression: timer-arm `drive()` must report Started so the consumer's
-    /// `in_op` flag flips; otherwise FilesChanged events skip buffering.
+    /// `in_op` flag flips; otherwise `FilesChanged` events skip buffering.
     #[test]
     fn cooldown_to_locked_when_lock_reappears_at_timer_fire() {
         let now = Instant::now();