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-ratatui-inline/tests/segment_differential.rs

@@ -14,7 +14,7 @@
 
 use kigi_ratatui_inline::split_into_line_segments;
 
-// ─── Reference: the previous termwiz-based implementation, verbatim ────────
+// Reference: the previous termwiz-based implementation, verbatim
 
 struct RefSegment<'a> {
     content: &'a str,
@@ -114,7 +114,7 @@ fn reference_split<'a>(input: &'a str, term_width: usize) -> Vec<RefSegment<'a>>
     segments
 }
 
-// ─── Comparison harness ─────────────────────────────────────────────────────
+// Comparison harness
 
 #[track_caller]
 fn assert_same(input: &str, widths: &[usize]) {
@@ -173,7 +173,8 @@ fn corpus_matches_reference() {
 /// Deterministic pseudo-random ANSI soup (xorshift, no extra deps).
 #[test]
 fn randomized_ansi_soup_matches_reference() {
-    let mut state = 0x243F_6A88_85A3_08D3_u64; // seed: pi digits
+    // seed: pi digits
+    let mut state = 0x243F_6A88_85A3_08D3_u64;
 
     let mut next = move || {
         state ^= state << 13;