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/ptyctl-cli/src/cli.rs

@@ -172,7 +172,7 @@ pub enum Commands {
         #[arg(long, group = "condition")]
         gone: Option<String>,
 
-        /// Wait until the screen has been unchanged for this many milliseconds
+        /// Wait until the screen has been `unchanged` for this many milliseconds
         #[arg(long, value_name = "MS", group = "condition")]
         stable_ms: Option<u64>,