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/build/kigi-proto-build/src/find_protoc.rs

@@ -24,22 +24,14 @@ fn is_github_actions() -> bool {
     env::var_os("GITHUB_ACTIONS").is_some()
 }
 
-/// Find `protoc` command.
+/// Locate `protoc`.
 ///
-/// Search order:
-/// 1. `$PROTOC` environment variable (set by Bazel `build_script_env` or user override)
-/// 2. `bin/protoc` walking up parent directories (dotslash wrapper for local dev)
-/// 3. `protoc` on `$PATH` (system install or other tooling)
-///
-/// When `bin/protoc` exists but fails to execute (e.g. the dotslash wrapper running
-/// in Bazel remote execution where `dotslash` is not installed), the error is not fatal —
-/// we fall through to the PATH-based lookup instead.
-///
-/// Returns `Ok(None)` if not found and not in a strict environment (GitHub Actions).
+/// Search order: `$PROTOC`, then `bin/protoc` walking parents (dotslash
+/// wrapper), then `$PATH`. A non-executable `bin/protoc` (e.g. dotslash
+/// missing under Bazel remote execution) is non-fatal — lookup continues
+/// on `$PATH`. Returns `Ok(None)` when missing outside GitHub Actions.
 pub fn find_protoc() -> anyhow::Result<Option<PathBuf>> {
-    // 1. Check the PROTOC env var first. This is the standard override used by prost-build
-    //    and is set by Bazel cargo_build_script build_script_env to point at a hermetic
-    //    protoc binary instead of the dotslash wrapper.
+    // `$PROTOC` is the prost-build override; Bazel sets it to a hermetic binary.
     if let Ok(protoc_env) = env::var("PROTOC") {
         let protoc = PathBuf::from(&protoc_env);
         if protoc.try_exists()? {
@@ -48,20 +40,17 @@ pub fn find_protoc() -> anyhow::Result<Option<PathBuf>> {
         }
     }
 
-    // 2. Walk up directories looking for bin/protoc (dotslash wrapper).
     let cwd = env::current_dir()?;
     let mut dir = cwd.clone();
     let mut dir_rel = PathBuf::new();
     loop {
-        // Return relative path to make build more deterministic.
+        // Relative path keeps cargo rerun fingerprints stable across machines.
         let protoc = dir_rel.join("bin/protoc");
         if protoc.try_exists()? {
             match check_protoc_good(&protoc) {
                 Ok(()) => return Ok(Some(protoc)),
                 Err(e) => {
-                    // bin/protoc exists but can't execute — likely the dotslash wrapper
-                    // in an environment without dotslash (e.g. Bazel remote execution).
-                    // Fall through to PATH-based lookup below.
+                    // Dotslash wrapper present but not runnable — try PATH next.
                     eprintln!(
                         "bin/protoc found at `{}` but failed to execute: {e:#}; \
                          trying protoc from PATH as fallback",
@@ -77,12 +66,10 @@ pub fn find_protoc() -> anyhow::Result<Option<PathBuf>> {
         dir_rel.push("..");
     }
 
-    // 3. Try protoc from PATH (system install or other tooling).
     if check_protoc_good(Path::new("protoc")).is_ok() {
         return Ok(Some(PathBuf::from("protoc")));
     }
 
-    // 4. Not found anywhere.
     if is_github_actions() {
         return Err(anyhow::anyhow!(
             "`protoc` not found (checked $PROTOC env, bin/protoc, and PATH)"