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-tui/src/test_util.rs

@@ -58,6 +58,8 @@ impl EnvVarGuard {
     /// returning a guard that restores the original on drop.
     pub fn set(key: &'static str, value: impl AsRef<std::ffi::OsStr>) -> Self {
         let original = std::env::var_os(key);
+        // SAFETY: single-threaded test execution for this guard's callers;
+        // no concurrent env access races with the mutation.
         unsafe {
             std::env::set_var(key, value);
         }
@@ -66,6 +68,7 @@ impl EnvVarGuard {
 }
 impl Drop for EnvVarGuard {
     fn drop(&mut self) {
+        // SAFETY: single-threaded test execution; see `set` above.
         unsafe {
             if let Some(value) = &self.original {
                 std::env::set_var(self.key, value);