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/benches/pty_bench.rs

@@ -135,7 +135,8 @@ async fn run() -> Result<ExitCode> {
 
         let res = scenario.run(&mut harness, &content).await;
 
-        // Best-effort cleanup regardless of scenario outcome.
+        // Cleanup runs regardless of scenario outcome, so failures are reported
+        // below rather than leaking a live PTY.
         let _ = harness.quit();
 
         match res {
@@ -160,7 +161,6 @@ async fn run() -> Result<ExitCode> {
         }
     }
 
-    // Emit JSON to stdout for downstream consumption.
     let json = serde_json::to_string_pretty(&results).context("serialize results")?;
     println!("{json}");