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-paths/src/lib.rs

@@ -32,7 +32,7 @@ pub trait ToAbsPath {
 
 /// Convert an absolute path to relative by stripping the root prefix.
 ///
-/// Returns the path unchanged if not under `root`. For strict validation,
+/// Returns the path `unchanged` if not under `root`. For strict validation,
 /// use [`RelPathBuf::from_absolute`] instead.
 pub fn to_relative_path(root: &Path, abs_path: &Path) -> PathBuf {
     abs_path