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-pty-harness/tests/scroll_matrix_curated.rs

@@ -89,7 +89,7 @@ async fn c1_auto_g8_midstream() {
     assert_cell_passes("c1_auto_g8_midstream").await;
 }
 
-/// The formerly-declared bug: the finalize-decel fix landed, so the G4 jerk
+/// The once-declared bug: the finalize-decel fix landed, so the G4 jerk
 /// cell passes outright — I-SMOOTH-COAST (post-input motion at most one
 /// tapered cap) and I-NO-DROP (finalize discards nothing) moved from the
 /// xfail set to ordinary pass rows. The cell id keeps its historical name