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-shell/tests/trace_replay.rs

@@ -34,7 +34,8 @@ const CANONICAL_FIXTURES: &[&str] = &[
 #[derive(Debug, Deserialize)]
 struct Fixture {
     name: String,
-    #[allow(dead_code)] // human-readable; surfaced only on assertion failure
+    // human-readable; surfaced only on assertion failure
+    #[allow(dead_code)]
     description: String,
     turns: Vec<Turn>,
 }
@@ -42,7 +43,8 @@ struct Fixture {
 #[derive(Debug, Deserialize)]
 #[serde(tag = "kind", rename_all = "snake_case")]
 enum Turn {
-    #[allow(dead_code)] // user turns are walked but never gate-evaluated
+    // user turns are walked but never gate-evaluated
+    #[allow(dead_code)]
     User(UserTurn),
     Assistant(AssistantTurn),
 }
@@ -57,7 +59,8 @@ struct UserTurn {
 #[derive(Debug, Deserialize)]
 struct AssistantTurn {
     turn_index: usize,
-    #[allow(dead_code)] // present for fixture clarity; the gate does not consult it
+    // present for fixture clarity; the gate does not consult it
+    #[allow(dead_code)]
     tool_calls_emitted: Vec<serde_json::Value>,
     todo_state_after_turn: Vec<TodoSnapshot>,
     backing_task_count: usize,
@@ -70,7 +73,8 @@ struct AssistantTurn {
 
 #[derive(Debug, Deserialize)]
 struct TodoSnapshot {
-    #[allow(dead_code)] // id is preserved for fixture readability
+    // id is preserved for fixture readability
+    #[allow(dead_code)]
     id: String,
     status: TodoStatus,
     content: String,