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/src/lib.rs

@@ -129,7 +129,7 @@ pub fn render_markdown_ratatui_with_buffers_width(
 /// still-open fenced code block: only the streaming tail re-render passes
 /// `Some(cache)`; `finish()` and non-streaming callers pass `None`. Everything
 /// other than that one open block (closed code blocks, HTML, math, tables,
-/// inline) always goes through the unchanged batch highlighter, so output is
+/// inline) always goes through the `unchanged` batch highlighter, so output is
 /// byte-for-byte identical to the cache-less path. See [`open_code_highlighter`].
 #[allow(clippy::too_many_arguments)]
 pub(crate) fn render_markdown_ratatui_with_link_id(