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/mod.rs

@@ -1,12 +1,11 @@
 //! Layer-2 stream transforms: turn raw HTTP chunk streams into
 //! [`SamplingEvent`](crate::events::SamplingEvent) streams.
 //!
-//! Each backend has its own transform because the raw chunk types
-//! differ; backend dispatch happens in M4's
-//! [`actor::request_task`](crate::actor::request_task), which knows
-//! the API backend from `SamplerConfig.api_backend` and calls the
-//! matching `SamplingClient::conversation_stream*` method before
-//! handing the result to the corresponding transform here.
+//! Each backend needs its own transform because the raw chunk types differ.
+//! Dispatch lives in [`actor::request_task`](crate::actor::request_task),
+//! which reads the backend from `SamplerConfig.api_backend`, calls the
+//! matching `SamplingClient::conversation_stream*` method, and hands the
+//! result to the transform here.
 
 pub mod chat_completions;
 pub mod collect;