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-fsnotify/benches/startup.rs

@@ -3,7 +3,7 @@
 //! All scenarios build ~12k total dirs so inotify-watch creation cost is
 //! comparable across them:
 //!
-//! - `favorable` — most dirs live in a gitignored `target/` the new code skips.
+//! - `favorable` — most dirs live in a gitignored `target/` that fan-out skips.
 //! - `fanout_w48_with_target` — 48 non-ignored top-level children PLUS a
 //!   gitignored `target/`: a realistic moderate-width repo that fans out and
 //!   skips the build dir (net win).
@@ -38,8 +38,8 @@ fn make_dirs(base: &Path, count: usize) {
     }
 }
 
-/// Favorable: three watched subtrees plus a large gitignored `target/` holding
-/// ~2/3 of the dirs. Kept at ~`TOTAL_DIRS` for comparability with the others.
+/// Three watched subtrees plus a large gitignored `target/` holding ~2/3 of
+/// the dirs, totalling ~`TOTAL_DIRS` for comparability with the other shapes.
 fn build_favorable_tree() -> TempDir {
     let temp = TempDir::new().unwrap();
     let root = temp.path();