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-ratatui-inline/src/tests.rs

@@ -227,7 +227,7 @@ mod links {
         // A small inline viewport near the bottom of the screen, grown to full
         // height, must scroll the rows it will cover up into native scrollback
         // (append_lines) instead of overwriting them. Regression guard for the
-        // previously-commented-out scroll_up in set_viewport_height's grow path
+        // earlier-commented-out scroll_up in set_viewport_height's grow path
         // (the overlay host depends on this in minimal mode).
         let mut t = Terminal::with_options(
             RecordingBackend::default(),