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-shell/src/session/two_pass.rs

@@ -18,7 +18,6 @@ const TWO_PASS_MIN_SUMMARY_BLOCK_CHARS: usize = 1000;
 /// Cap on NOTE₁ text embedded in pass2 (carrier + special turn).
 const TWO_PASS_MAX_NOTE1_CHARS: usize = 12_000;
 
-/// Result of splitting a conversation for two-pass compaction.
 #[derive(Debug, Clone, Copy)]
 pub(crate) struct TwoPassSplit<'a> {
     pub prefix: &'a [ConversationItem],
@@ -287,7 +286,8 @@ mod tests {
         // split (pass2 needs recent turns to rewrite against NOTE₁).
         let weights = vec![10u64; 40];
         let idx = split_index_by_token_fraction(&weights, TWO_PASS_DEFAULT_SPLIT_FRACTION);
-        assert_eq!(idx, 38); // 38/40 = 95% by weight
+        // 38/40 = 95% by weight
+        assert_eq!(idx, 38);
         assert!(idx < weights.len());
     }