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/views/completion_dropdown.rs

@@ -34,7 +34,8 @@ pub fn dropdown_height(state: &CompletionDropdownState) -> u16 {
         return 0;
     }
     let item_rows = (state.items.len() as u16).min(MAX_VISIBLE_ROWS);
-    1 + item_rows // separator + items
+    // separator + items
+    1 + item_rows
 }
 
 /// Compute the scroll offset so the selected row stays centred.
@@ -269,7 +270,8 @@ mod tests {
             ],
             ..Default::default()
         };
-        assert_eq!(dropdown_height(&state), 3); // 1 separator + 2 items
+        // 1 separator + 2 items
+        assert_eq!(dropdown_height(&state), 3);
     }
 
     #[test]
@@ -404,7 +406,8 @@ mod tests {
         assert!(!state.open);
         assert_eq!(state.selected, 0);
         assert!(state.hovered.is_none());
-        assert_eq!(state.generation, 5); // generation preserved
+        // generation preserved
+        assert_eq!(state.generation, 5);
         assert!(state.items.is_empty());
         // Anchor left in place (inert without items); the next landing
         // overwrites it atomically with the new items.