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-tool-protocol/tests/jsonrpc_envelope.rs

@@ -202,7 +202,6 @@ fn jsonrpc_id_round_trips_to_request_id_correlator() {
     let envelope_id = JsonRpcId::from_request_id(&original);
     assert_eq!(envelope_id.as_request_id().unwrap(), original);
 
-    // Numeric ids are stringified.
     let nid = JsonRpcId::Number(7);
     assert_eq!(nid.as_request_id().unwrap().as_str(), "7");
 }
@@ -234,10 +233,9 @@ fn full_call_envelope_serialises_to_expected_shape() {
     assert_eq!(v["params"]["tool_call_id"], json!("call_xyz"));
 }
 
-/// The envelope-level `session_id` and an inner `params.session_id` (e.g.
-/// on `ToolsListParams`) are independent keys in the wire JSON tree.
-/// This test pins that invariant so a refactor that accidentally
-/// collapses the two layers (e.g. via `#[serde(flatten)]`) fails loudly.
+/// The envelope-level `session_id` and an inner `params.session_id` are
+/// independent keys in the wire JSON tree; a refactor that collapses the two
+/// layers (e.g. via `#[serde(flatten)]`) must fail here.
 #[test]
 fn envelope_session_id_and_inner_params_session_id_are_distinct_layers() {
     use kigi_tool_protocol::{ToolDefinitionMode, ToolsListParams};