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-codebase-graph/src/manager/builder.rs

@@ -235,7 +235,8 @@ impl IndexBuilder {
             .git_ignore(self.respect_gitignore)
             .git_global(self.respect_gitignore)
             .git_exclude(self.respect_gitignore)
-            .threads(self.num_threads.min(12)) // Use parallel walking (capped at 12)
+            // Use parallel walking (capped at 12)
+            .threads(self.num_threads.min(12))
             .build_parallel();
 
         walker.run(|| {
@@ -309,7 +310,6 @@ impl IndexBuilder {
         //
         // New approach: for each batch of build_batch_size files:
         //   1. Parse in parallel (par_chunks preserves thread-local cache locality)
-        //   2. Merge the batch into the index
         //   3. Drop the batch before starting the next one
         // Peak = O(build_batch_size) symbols + growing index simultaneously.
         for batch in file_paths.chunks(build_batch_size) {