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/src/stream/chat_completions.rs

@@ -96,7 +96,8 @@ pub fn stream_chat_completions<'a>(
         loop {
             let next = match tokio::time::timeout(idle_timeout, stream.next()).await {
                 Ok(Some(next)) => next,
-                Ok(None) => break, // stream ended normally
+                // stream ended normally
+                Ok(None) => break,
                 Err(_elapsed) => {
                     let err = SamplingError::IdleTimeout {
                         elapsed_secs: idle_timeout.as_secs(),
@@ -253,7 +254,7 @@ pub fn stream_chat_completions<'a>(
             }
         }
 
-        // ── Build the final response ─────────────────────────────────
+        // Build the final response
         let tool_calls: Vec<ToolCall> = tool_call_acc
             .into_values()
             .map(|(id, name, arguments)| {