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-version/src/lib.rs

@@ -10,7 +10,7 @@ pub const VERSION: &str = match option_env!("KIGI_VERSION") {
 };
 
 /// [`TEST_VERSION_ENV`] override first, then [`VERSION`]. Trimmed so
-/// non-semver-aware callers can pass the result straight into parsing.
+/// callers can pass the result straight into semver parsing.
 pub fn installed() -> String {
     std::env::var(TEST_VERSION_ENV)
         .map(|v| v.trim().to_string())
@@ -21,21 +21,12 @@ pub fn installed_semver() -> Result<Version, semver::Error> {
     Version::parse(&installed())
 }
 
-/// Format the compiled version with a channel label for user-facing display.
-///
-/// `channel_label` is a pre-formatted suffix such as `" [alpha]"`, `" [stable]"`,
-/// or `""` (empty when no cached pointer is available). Obtain it from
-/// `kigi_update::channel_label()`.
-///
-/// Example: `"0.2.5 [stable]"` or `"0.2.5 [alpha]"`.
+/// `channel_label` is a pre-formatted suffix such as `" [alpha]"`, or `""` when
+/// no cached pointer is available. Obtain it from `kigi_update::channel_label()`.
 pub fn display_version(channel_label: &str) -> String {
     format!("{}{}", VERSION, channel_label)
 }
 
-/// Format a version-with-commit string with a channel label.
-///
-/// Same semantics as [`display_version`] but for the full
-/// `"0.2.5 (abc1234)"` string.
 pub fn display_version_with_commit(version_with_commit: &str, channel_label: &str) -> String {
     format!("{}{}", version_with_commit, channel_label)
 }
@@ -44,8 +35,6 @@ pub fn display_version_with_commit(version_with_commit: &str, channel_label: &st
 mod tests {
     use super::*;
 
-    /// Display formatting invariant matrix — verifies label appending
-    /// works correctly across all label states (alpha, stable, empty).
     #[test]
     fn test_display_version_formatting_matrix() {
         let cases: &[(&str, &str, &str)] = &[
@@ -68,7 +57,7 @@ mod tests {
                 label,
             );
         }
-        // display_version uses compiled VERSION — just verify the label appends
+        // display_version reads the compiled VERSION, so only the suffix is assertable.
         assert_eq!(display_version(""), VERSION);
         assert!(display_version(" [stable]").ends_with("[stable]"));
     }