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-log/src/memory_log.rs

@@ -1,15 +1,8 @@
 //! Memory system tracing target and optional file-based logging layer.
 //!
-//! Provides a dedicated tracing target (`xai_memory`) with an optional
-//! file logger that writes to `~/.kigi/logs/memory.log`.
-//!
-//! ## When to use
-//!
-//! Use `tracing::info!(target: memory_log::TARGET, ...)` at memory system
-//! lifecycle points — config resolution, storage init, flush, search, etc.
-//! These events are always emitted (zero cost when the layer is absent).
-//!
-//! ## Enabling (debug builds)
+//! Emit events with `tracing::info!(target: memory_log::TARGET, ...)` at memory
+//! system lifecycle points — config resolution, storage init, flush, search.
+//! They are always emitted, at zero cost when the layer is absent.
 //!
 //! ```bash
 //! # build with memory logging enabled, then:
@@ -63,9 +56,7 @@ mod inner {
         }
     }
 
-    /// Build the memory log layer.
-    ///
-    /// Writes to `~/.kigi/logs/memory.log`. Filters to `xai_memory=trace`.
+    /// Writes to `~/.kigi/logs/memory.log`, filtered to `xai_memory=trace`.
     /// Set `KIGI_MEMORY_LOG=0` to disable, `KIGI_MEMORY_LOG=/path` to redirect.
     pub fn layer<S>() -> Option<impl Layer<S>>
     where