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-sampler/tests/support/mod.rs

@@ -16,8 +16,9 @@ pub fn test_config(base_url: &str, api_key: &str) -> SamplerConfig {
     }
 }
 
-/// Drive one POST through the client; the canned `{}` body is not a valid
-/// completion, but only the wire-level request matters here.
+/// Drive one POST through the client. The response is discarded: the counting
+/// server never returns a valid completion, and only the wire-level request
+/// matters here.
 pub async fn send_one(client: &SamplingClient) {
     let request = ConversationRequest {
         items: vec![ConversationItem::User(UserItem {