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-tui/src/tips/render.rs

@@ -104,7 +104,8 @@ mod tests {
         buf.set_string(0, 0, "XXXXXXXX", Style::default());
         buf.set_string(0, 1, "XXXXXXXX", Style::default());
 
-        let line = Line::from("0123456789"); // wider than the rect
+        // Wider than the rect.
+        let line = Line::from("0123456789");
         render_ephemeral_tip(area, &mut buf, &line);
 
         assert_eq!(row_text(&buf, area, 0), "01234567", "truncated at width");