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/syntax.rs

@@ -1,8 +1,8 @@
 //! Syntax highlighting initialization.
 //!
-//! Provides lazily-initialized `Syntect` instances for code highlighting.
-//! Dark themes (KigiNight, TokyoNight) share `kigi-night.tmTheme`;
-//! KigiDay uses `kigi-day.tmTheme` with deepened colors for light backgrounds.
+//! One lazily-initialized `Syntect` per tmTheme, shared by every `ThemeKind`
+//! that maps to it. KigiDay needs its own because `kigi-day.tmTheme` deepens
+//! the palette for light backgrounds.
 
 use std::sync::OnceLock;
 
@@ -14,7 +14,6 @@ static SYNTECT_KIGINIGHT: OnceLock<Syntect> = OnceLock::new();
 static SYNTECT_TOKYONIGHT: OnceLock<Syntect> = OnceLock::new();
 static SYNTECT_KIGIDAY: OnceLock<Syntect> = OnceLock::new();
 
-/// Convert syntect style to ratatui foreground-only style, quantized for terminal color support.
 pub fn syntect_to_ratatui_fg(style: syntect::highlighting::Style) -> ratatui::style::Style {
     let fg = crate::theme::quantize(ratatui::style::Color::Rgb(
         style.foreground.r,
@@ -35,7 +34,6 @@ pub fn syntect_to_ratatui_fg(style: syntect::highlighting::Style) -> ratatui::st
     out
 }
 
-/// Highlight a single line of source, falling back to plain text style.
 pub fn highlight_line(
     text: &str,
     highlighter: &mut Option<syntect::easy::HighlightLines<'_>>,
@@ -63,7 +61,6 @@ pub fn highlight_line(
     vec![ratatui::text::Span::styled(text.to_string(), fallback)]
 }
 
-/// Returns the syntect instance matching the active theme.
 pub fn get_syntect() -> &'static Syntect {
     match crate::theme::Theme::current_kind() {
         ThemeKind::KigiNight