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-log/src/session_ctx.rs

@@ -5,16 +5,13 @@
 //! `session_id` field. [`with_session_ctx`] installs that span for the
 //! duration of a session's work.
 
-/// The `session_id` field name the debug-log firehose router keys on:
-/// `debug_log::SessionIdVisitor` stashes a `SessionId` extension on any span
-/// carrying this field — the span *name* is not load-bearing for routing. Shared
-/// so the `info_span!` here and the router in `debug_log` can't silently drift; a
-/// rename trips `session_span_exposes_router_field` below.
+/// The field name the firehose router keys on: `debug_log::SessionIdVisitor`
+/// stashes a `SessionId` extension on any span carrying this field — the span
+/// *name* is not load-bearing for routing.
 pub(crate) const SESSION_ID_FIELD: &str = "session_id";
 
-/// Build the per-session tracing span the firehose router routes by. The field
-/// name MUST be the literal `session_id` (tracing field names can't come from a
-/// const); the test below pins it against [`SESSION_ID_FIELD`].
+/// The field name MUST be the literal `session_id` (tracing field names can't
+/// come from a const); the test below pins it against [`SESSION_ID_FIELD`].
 fn session_span(session_id: &str) -> tracing::Span {
     tracing::info_span!("session", session_id = %session_id)
 }
@@ -30,11 +27,8 @@ pub async fn with_session_ctx<F: std::future::Future>(session_id: &str, fut: F)
 mod tests {
     use super::*;
 
-    /// The debug-log firehose router (`debug_log`) finds the session span by its
-    /// `session_id` field (not by name). That field name is a literal in
-    /// `session_span` (tracing field names can't be a const), so pin it against the
-    /// shared const here — a rename of either breaks this test instead of silently
-    /// degrading routing to the per-pid fallback.
+    /// Without this pin, a diverging field name silently degrades routing to the
+    /// per-pid fallback instead of failing.
     #[test]
     fn session_span_exposes_router_field() {
         // A bare registry enables every callsite, so the span has live metadata.