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/messages.rs

@@ -241,7 +241,8 @@ pub fn stream_messages<'a>(
                             arguments_delta: None,
                         };
                     }
-                    _ => {} // Image / ToolResult are not expected in assistant streams.
+                    // Image / ToolResult are not expected in assistant streams.
+                    _ => {}
                 },
 
                 MessageStreamEvent::ContentBlockDelta { index, delta } => {
@@ -460,7 +461,7 @@ pub fn stream_messages<'a>(
             return;
         }
 
-        // ── Build the final response ─────────────────────────────────
+        // Build the final response
         let model_id = final_model.unwrap_or_default();
         // Match the OAI Responses convention: prompt_tokens = full prompt, cached_prompt_tokens = cache hits only.
         let total_prompt_tokens = final_input_tokens