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-mermaid/tests/pure_engine.rs

@@ -76,7 +76,7 @@ fn sequence_diagram_with_activations_renders_to_png() {
 
 /// Regression: a class diagram using quoted cardinalities, stereotypes,
 /// generics, and the full relation set must render instead of erroring
-/// (previously failed with "Unrecognized classDiagram line" on
+/// (earlier failed with "Unrecognized classDiagram line" on
 /// `Owner "1" o-- "0..*" Animal`).
 #[test]
 fn class_diagram_with_cardinalities_renders() {
@@ -149,7 +149,7 @@ fn long_identifier_node_labels_survive_intact_in_svg() {
 
 /// A categorical-x-axis xychart with two `line` series must render to a decodable
 /// PNG through the full `[Open Image]` path (source -> SVG -> raster), on both
-/// themes. The categorical x-axis (no `-->`) previously failed to open.
+/// themes. The categorical x-axis (no `-->`) earlier failed to open.
 #[test]
 fn categorical_xychart_with_two_series_renders_to_png() {
     const SOURCE: &str = "xychart-beta\n    \