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/test_install_internal.rs

@@ -31,9 +31,7 @@ fn base(server: &MockServer) -> String {
     format!("{}/releases", server.uri())
 }
 
-// ─────────────────────────────────────────────────────────────────────────────
 // Happy-path
-// ─────────────────────────────────────────────────────────────────────────────
 
 #[tokio::test]
 #[serial]
@@ -207,9 +205,7 @@ async fn install_removes_legacy_links() {
     assert!(bin_dir.join("kigi").is_symlink(), "kigi link installed");
 }
 
-// ─────────────────────────────────────────────────────────────────────────────
 // Failure paths
-// ─────────────────────────────────────────────────────────────────────────────
 
 #[tokio::test]
 #[serial]
@@ -447,10 +443,8 @@ async fn install_swap_failure_leaves_prior_install_active() {
     );
 }
 
-// ─────────────────────────────────────────────────────────────────────────────
 // Rollback semantics: pinned installs move DOWN as well as up (the release
 // channel is authoritative for the internal installer).
-// ─────────────────────────────────────────────────────────────────────────────
 
 #[tokio::test]
 #[serial]
@@ -479,9 +473,7 @@ async fn install_rollback_then_upgrade_sequence() {
     }
 }
 
-// ─────────────────────────────────────────────────────────────────────────────
 // Cleanup integration: install v1..v3, verify N-1 retention.
-// ─────────────────────────────────────────────────────────────────────────────
 
 #[tokio::test]
 #[serial]