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-chat-state/src/persistence.rs

@@ -27,9 +27,7 @@ pub trait ChatPersistence: Send + 'static {
     fn flush(&mut self);
 }
 
-// ============================================================================
 // Mock (test double) — channel-based, no locks, no atomics
-// ============================================================================
 
 /// A record of a persistence call, sent over a channel to the test.
 #[derive(Debug, Clone)]
@@ -101,9 +99,7 @@ impl ChatPersistence for MockChatPersistence {
     }
 }
 
-// ============================================================================
 // Null (noop) — for benchmarks / scenarios where persistence is unwanted
-// ============================================================================
 
 /// No-op implementation: discards everything (for benchmarks / noop scenarios).
 pub struct NullChatPersistence;