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-chat-state/src/compaction_transcript.rs

@@ -77,7 +77,7 @@ pub const INDEX_HEADER: &str = "# Compaction Segment Index\n\n\
      | Segment | File | Turns | Approx bytes | Keywords |\n\
      |---|---|---|---|---|\n";
 
-/// Zero-padded segment number, e.g. `007`. The single source of the pad width.
+/// Zero-`padded` segment number, e.g. `007`. The single source of the pad width.
 fn segment_label(index: u64) -> String {
     format!("{index:03}")
 }
@@ -724,7 +724,7 @@ mod tests {
         assert_eq!(classify_compaction_path("compaction/notes.md"), None);
     }
 
-    // --- Parity with the Python implementation's own test vectors (compaction_utils_test.py) ---
+    // Parity with the Python implementation's own test vectors (compaction_utils_test.py)
 
     /// Keyword extraction: the Python `TestExtractKeywords` vectors (bare `8.`
     /// headers, stopword filtering, dedup, no-section-8 fallback) plus our