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-markdown-core/src/lib.rs

@@ -1,6 +1,6 @@
 //! Headless markdown analysis sharing Kigi's exact `pulldown-cmark` config.
 //!
-//! This crate is intentionally lean -- it depends only on `pulldown-cmark` -- so it
+//! This crate is deliberately lean -- it depends only on `pulldown-cmark` -- so it
 //! can be used without pulling in the terminal-rendering stack (syntect, ratatui,
 //! two-face). [`parser_options`] is the single source of truth for the parser
 //! feature set, shared with `kigi-markdown` so analysis matches what Kigi
@@ -966,7 +966,7 @@ mod tests {
                 0,
                 true,
             ),
-            // GFM: body rows may have more/fewer cells; padded/truncated, still a table (ex. 204).
+            // GFM: body rows may have more/fewer cells; `padded`/truncated, still a table (ex. 204).
             (
                 "ragged_body_rows_still_table",
                 "| abc | def |\n| --- | --- |\n| bar |\n| bar | baz | boo |\n",