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-tools/src/implementations/read_file/pptx.rs

@@ -138,7 +138,6 @@ fn extract_drawingml_text(xml: &str) -> Result<String, String> {
             }
             Ok(Event::End(ref e)) => match e.local_name().as_ref() {
                 b"t" => in_text_run = false,
-                // End of paragraph: line break.
                 b"p" if !text.is_empty() && !text.ends_with('\n') => text.push('\n'),
                 _ => {}
             },