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/modal_window_state.rs

@@ -18,7 +18,7 @@ pub struct ModalWindowState {
     /// Full popup area (for click-outside-to-close detection).
     pub popup_area: Option<Rect>,
 
-    // -- Tabs (optional) --
+    // Tabs (optional)
     /// Currently active tab index.
     pub active_tab: usize,
     /// Number of tabs (0 = no tab bar).
@@ -28,7 +28,7 @@ pub struct ModalWindowState {
     /// Whether the tab bar region has keyboard focus. When true, Left/Right
     pub tabs_focused: bool,
 
-    // -- Footer shortcuts --
+    // Footer shortcuts
     /// Hit-test areas for clickable footer shortcuts.
     pub shortcut_hits: Vec<ShortcutHitArea>,
     /// Which footer shortcut (by index) is currently hovered.