Split canonical ReasoningEffort::Max out of Xhigh (providers P0c-1)

OpenAI (Responses) and Anthropic (Messages) treat xhigh and max as
DISTINCT effort levels in 2026, and the Kimi K3 wire's top tier is max —
the old parse alias (max→Xhigh) conflated them. Canonical Max now exists:
parse/as_str/serde split, Messages mapping sends xhigh and max as their
own tokens (was Xhigh→"max"), and the K3 menu token max carries
canonical Max end to end.

Kimi wire is byte-identical in all four flows (menu pick, restored
legacy xhigh session, --reasoning-effort flag, /effort command) —
adversarially traced and pinned: kimi_compat's string-level xhigh→max
rename covers legacy tokens, max passes through verbatim.

From the review:
- Rollback safety: persisted reasoning_effort (session summaries, chat
  history) deserializes leniently — unknown future tokens degrade to
  None with a warning instead of hiding sessions or failing resume.
- Restore migration: a pre-split xhigh override onto a model whose menu
  offers max but not xhigh (K3) migrates once, healing display/active-row
  drift and re-persisting the live vocabulary.
- /effort max now rejects (with the offered list) on models whose menu
  lacks a max row instead of silently applying xhigh; deliberate, tested.
- The interim Responses-backend Max→xhigh downgrade (async-openai has no
  Max variant through 0.41) warns loudly; real max wiring lands with the
  OpenAI provider cycle via post-serialize body patch.
- Two rusted ignored-e2e wire pins asserted the pre-adapt reasoning_effort
  key (deleted by the body adapter since ea0ce9d); they now pin the real
  thinking.effort=max shape.

crates/codegen/kigi-tui/tests/pty_e2e/reasoning_efforts_menu_renders_and_remaps_on_wire.rs

@@ -69,13 +69,16 @@ async fn reasoning_efforts_menu_renders_and_remaps_on_wire() {
         .wait_for_text("second turn", Duration::from_secs(30))
         .expect("second turn rendered");
 
-    let sent_xhigh = content
+    // The chat-completions body is always adapted before send: the
+    // `reasoning_effort` scalar is folded into Kimi's `thinking` field and
+    // canonical `xhigh` is spelled `max` on the wire (kimi_compat).
+    let sent_effort = content
         .request_bodies()
         .iter()
-        .any(|b| b.pointer("/reasoning_effort").and_then(|v| v.as_str()) == Some("xhigh"));
+        .any(|b| b.pointer("/thinking/effort").and_then(|v| v.as_str()) == Some("max"));
     assert!(
-        sent_xhigh,
-        "`/effort deep` must send the mapped canonical reasoning_effort=xhigh\nbodies: {:#?}",
+        sent_effort,
+        "`/effort deep` must send the remapped level as thinking.effort=max\nbodies: {:#?}",
         content.request_bodies()
     );