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-acp-lib/src/line_reader.rs

@@ -126,7 +126,7 @@ impl AsyncRead for LineBufferedRead {
                 Poll::Ready(Ok(n))
             }
             Poll::Ready(Some(Err(e))) => Poll::Ready(Err(e)),
-            Poll::Ready(None) => Poll::Ready(Ok(0)), // EOF
+            Poll::Ready(None) => Poll::Ready(Ok(0)),
             Poll::Pending => Poll::Pending,
         }
     }
@@ -146,7 +146,7 @@ async fn read_line_capped(
         let (consumed, done) = {
             let available = reader.fill_buf().await?;
             if available.is_empty() {
-                return Ok(buf.len()); // EOF
+                return Ok(buf.len());
             }
             match available.iter().position(|&b| b == b'\n') {
                 Some(pos) => {
@@ -283,15 +283,12 @@ mod tests {
             let mut reader = LineBufferedRead::spawn_local(source);
             let mut small_buf = [0u8; 3];
 
-            // First read: "abc"
             let n = reader.read(&mut small_buf).await.unwrap();
             assert_eq!(&small_buf[..n], b"abc");
 
-            // Second read: "def"
             let n = reader.read(&mut small_buf).await.unwrap();
             assert_eq!(&small_buf[..n], b"def");
 
-            // Third read: "\n"
             let n = reader.read(&mut small_buf).await.unwrap();
             assert_eq!(&small_buf[..n], b"\n");