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/debug_log.rs

@@ -105,7 +105,7 @@ where
     Ok(fmt_layer)
 }
 
-// ── Per-session routing layer ───────────────────────────────────────────────
+// Per-session routing layer
 
 /// Filesystem-safe session key. Sanitized once at capture (`on_new_span`) and
 /// stashed in the span's tracing extensions, so events fired anywhere under the
@@ -148,7 +148,7 @@ impl Visit for EventVisitor {
     }
 }
 
-// Format one compact, ANSI-free firehose line. Intentionally NOT byte-identical
+// Format one compact, ANSI-free firehose line. Deliberately NOT byte-identical
 // to `fmt::Layer`: its `FormatEvent` can't be reused from another layer and a
 // `MakeWriter` can't see span context, so we render here. Span context is
 // omitted on purpose — the file name already carries the session id.
@@ -355,7 +355,7 @@ where
     }
 }
 
-// ── Install + lifecycle ──────────────────────────────────────────────────────
+// Install + lifecycle
 
 /// Resolve the requested debug target and install the matching firehose layer on
 /// `registry`, then init the subscriber.