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/theme/terminal_default.rs

@@ -4,7 +4,7 @@
 //! the terminal's own canvas it can land dark-on-dark or light-on-light
 //! (e.g. macOS in Light Mode + a dark terminal profile). Polarity detection
 //! is not reliable either: OS appearance and OSC 11 both disagree with the
-//! actual canvas in edge cases and can change mid-session. Terminal
+//! actual canvas in edge cases and can differ mid-session. Terminal
 //! profiles, however, tune their **default** fg/bg to be legible against
 //! their own background — this is how `git` and `ls` stay readable on any
 //! terminal — so a palette built from `Reset` (body) + sparse named ANSI-16