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-update/tests/common/artifact_server.rs

@@ -277,7 +277,7 @@ fn handle_connection(
     let range = parse_range(&request);
     let addr = stream.local_addr().unwrap();
 
-    // ── Metadata routes ─────────────────────────────────────────────────────
+    // Metadata routes
     if path == "/releases/latest" {
         let latest = state.lock().unwrap().latest.clone();
         let body = release_json(&addr, &latest);
@@ -290,7 +290,7 @@ fn handle_connection(
         return;
     }
 
-    // ── Asset routes: /dl/{version}/{name} ──────────────────────────────────
+    // Asset routes: /dl/{version}/{name}
     let Some(rest) = path.strip_prefix("/dl/") else {
         write_simple_response(&mut stream, "404 Not Found", b"not found", is_head);
         return;
@@ -330,7 +330,7 @@ fn handle_connection(
         return;
     }
 
-    // ── Archive body with corruption modes ──────────────────────────────────
+    // Archive body with corruption modes
     // Count only body-serving GETs; the parallel path's HEAD probe is excluded.
     if !is_head {
         gets.fetch_add(1, Ordering::Relaxed);