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/compaction_mode.rs

@@ -32,7 +32,7 @@ impl CompactionMode {
         }
     }
 
-    /// Replace the detail level if this is `Segments`, else unchanged. Lets the
+    /// Replace the detail level if this is `Segments`, else `unchanged`. Lets the
     /// resolver attach the separately-resolved `KIGI_COMPACTION_DETAIL`.
     pub fn with_segment_detail(self, detail: CompactionDetail) -> Self {
         match self {