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-acp-lib/src/stdin_reader.rs

@@ -138,7 +138,8 @@ fn isolate_process_stdin() -> Option<std::fs::File> {
     use std::os::windows::io::FromRawHandle as _;
 
     // Win32 constants (inlined to avoid a dependency).
-    const STD_INPUT_HANDLE: u32 = 0xFFFF_FFF6; // (DWORD)-10
+    // (DWORD)-10
+    const STD_INPUT_HANDLE: u32 = 0xFFFF_FFF6;
     const DUPLICATE_SAME_ACCESS: u32 = 0x0000_0002;
     const GENERIC_READ: u32 = 0x8000_0000;
     const FILE_SHARE_READ: u32 = 0x0000_0001;
@@ -186,7 +187,8 @@ fn isolate_process_stdin() -> Option<std::fs::File> {
             process,
             &mut duplicate,
             0,
-            0, // not inheritable
+            // not inheritable
+            0,
             DUPLICATE_SAME_ACCESS,
         ) == 0
         {