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/paste_latency.rs

@@ -252,7 +252,7 @@ async fn bench_cell(
     let mut harness = spawn_ready(binary, rows, cols, &content, surface)?;
 
     // Image mode keeps one PNG on the pasteboard for the whole cell (the
-    // pager re-reads the unchanged clipboard on every Ctrl+V).
+    // pager re-reads the `unchanged` clipboard on every Ctrl+V).
     let tmp = tempfile::tempdir().context("tempdir for the clipboard PNG")?;
     if mode == Mode::Image {
         let png = write_fixture_png(tmp.path())?;