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-shell/build.rs

@@ -10,7 +10,6 @@ use std::path::{Path, PathBuf};
 const RG_VER: &str = "15.0.0";
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    // Only bundle in release builds to avoid slowing down cargo check.
     println!("cargo:rerun-if-env-changed=KIGI_SHELL_BUNDLE_RG_PATH");
     println!("cargo:rerun-if-env-changed=KIGI_SHELL_RG_DOWNLOAD_BASE");
     // Declare our custom cfg to the compiler so cfg(bundle_rg) is recognized by lints