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-acp-lib/src/normalize.rs

@@ -33,7 +33,7 @@
 /// `\u2028` and surrogate pairs in text).
 ///
 /// Any line that fails both parses passes through byte-identical —
-/// deliberately: the acp crate keeps ownership of garbage handling.
+/// Deliberately: the acp crate keeps ownership of garbage handling.
 pub(crate) fn normalize_json_line(line: Vec<u8>) -> Vec<u8> {
     if !line.windows(2).any(|w| w == br"\/") {
         return line;