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-fast-worktree/src/db/mod.rs

@@ -138,7 +138,7 @@ impl WorktreeDb {
             .with_context(|| format!("failed to open worktree DB: {}", path.display()))?;
         let db = Self { conn };
         db.set_journal_mode(journal_mode)?;
-        // Normal statement timeout, now that the conversion budget is done.
+        // Normal statement timeout, because the conversion budget is done.
         db.conn
             .busy_timeout(std::time::Duration::from_millis(5000))?;
         db.init_schema()?;