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-tools/src/util/base64_images.rs

@@ -564,7 +564,8 @@ mod tests {
 
     #[test]
     fn strip_pdf_size_calculation() {
-        let pdf_b64 = payload(12288); // 12288 * 3/4 / 1024 = 9 KB
+        // 12288 * 3/4 / 1024 = 9 KB
+        let pdf_b64 = payload(12288);
         let input = format!("data:application/pdf;base64,{pdf_b64} end");
         let result = strip_pdf_data_uris(&input).unwrap();
         assert!(result.contains("[PDF attachment removed \u{2014} 9 KB]"));
@@ -764,8 +765,6 @@ mod tests {
         );
     }
 
-    // ─── try_extract_base64_images tests ──────────────────────────────
-
     #[test]
     fn try_extract_no_images_returns_none() {
         assert!(try_extract_base64_images("Plain text with no images.").is_none());