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/fuzz/fuzz_targets/render_all.rs

@@ -11,19 +11,16 @@ fuzz_target!(|data: &[u8]| {
         return;
     };
 
-    // Full render: pretty / non-pretty
     for pretty in [true, false] {
         let _ = render_markdown_ratatui_full(s, STYLE, pretty, None);
     }
 
-    // Streaming with rotating chunk sizes: pretty / non-pretty
     for pretty in [true, false] {
         let mut r = StreamingMarkdownRenderer::new(STYLE, pretty);
         let mut pos = 0;
         let mut ci = 0;
         while pos < s.len() {
             let mut end = (pos + CHUNK_SIZES[ci]).min(s.len());
-            // snap to char boundary
             while end < s.len() && !s.is_char_boundary(end) {
                 end += 1;
             }