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/common/kigi-test-utils/src/env.rs

@@ -1,8 +1,7 @@
 //! Environment-variable test knobs.
 
-/// Parse a `usize` env knob, falling back to `default` when unset or
-/// unparseable. The perf-repro convention for sizing `#[ignore]` benches
-/// (e.g. `KIGI_PERF_GIT_FILES`).
+/// Parse a `usize` env knob; use `default` when unset or unparseable.
+/// Perf-repro convention for sizing `#[ignore]` benches (e.g. `KIGI_PERF_GIT_FILES`).
 pub fn env_usize(key: &str, default: usize) -> usize {
     std::env::var(key)
         .ok()