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-workspace/src/error.rs

@@ -4,7 +4,7 @@ use crate::capability::CapabilityMode;
 
 /// Errors surfaced by the workspace public API.
 ///
-/// `#[non_exhaustive]` so adding new variants is a non-breaking change.
+/// `#[non_exhaustive]` so introducing new variants is a non-breaking extension.
 /// Tests should match on variants rather than scrape the `Display` text.
 #[derive(Debug, thiserror::Error)]
 #[non_exhaustive]