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-fast-worktree/src/mount_info.rs

@@ -15,7 +15,6 @@ use anyhow::{Context, Result};
 /// ```
 #[derive(Debug, Clone)]
 pub struct MountEntry {
-    /// Mount ID.
     #[allow(dead_code)]
     pub mount_id: u32,
     /// Parent mount ID.
@@ -243,7 +242,7 @@ pub fn is_fuse_mount(entries: &[MountEntry], path: &Path) -> bool {
     })
 }
 
-// ── Internal helpers ─────────────────────────────────────────────────────
+// Internal helpers
 
 /// Parse a single mountinfo line.
 fn parse_line(line: &str) -> Option<MountEntry> {