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-auth/src/visibility.rs

@@ -1,7 +1,7 @@
-/// Apply auth headers to outbound visibility requests.
-/// Implemented by `kigi-shell::util::kigi_auth_credentials::KigiAuthCredentials`
-/// to keep credential construction owned by shell while letting data-collector
-/// build the request without reaching back into shell types.
+/// Applies auth headers to outbound visibility requests. Implemented by
+/// `kigi-shell::util::kigi_auth_credentials::KigiAuthCredentials`, keeping
+/// credential construction owned by shell while data-collector builds the
+/// request without reaching back into shell types.
 pub trait HttpAuth: Send + Sync {
     fn apply(&self, builder: reqwest::RequestBuilder, base_url: &str) -> reqwest::RequestBuilder;
 }