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-shared/examples/clipboard_probe.rs

@@ -1,9 +1,8 @@
-//! Reproducible harness for benchmarking clipboard attachment reads
-//! (the paste hot path).
+//! Benchmark harness for the clipboard paste hot path.
 //!
-//! Runs one `get_attachments()` — the same unified probe the pager's paste
-//! pipeline executes — and prints the outcome plus wall time. Benchmark the
-//! native in-process read against the `osascript` fallback with hyperfine:
+//! Runs the same `get_attachments()` probe the pager's paste pipeline executes
+//! and prints the outcome plus wall time. Compare the native in-process read
+//! against the `osascript` fallback with hyperfine:
 //!
 //! ```text
 //! # put an image on the pasteboard first, e.g.: