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-shell/tests/test_built_binary_e2e.rs

@@ -1401,15 +1401,17 @@ async fn headless_reasoning_efforts_payload_parses_and_legacy_effort_rides_wire(
     assert_headless_success(&result, "kigi -p reasoning_efforts list", Some(&server));
     assert_no_crashes(&result.stderr);
 
-    // The legacy effort scalar rides the chat-completions request unchanged.
+    // The chat-completions body is always adapted before send: the effort
+    // scalar becomes Kimi's `thinking` field, with canonical `xhigh`
+    // spelled `max` on the wire (kimi_compat rename).
     let effort_on_wire = server.requests().iter().any(|r| {
         r.body.as_ref().is_some_and(|body| {
-            body.pointer("/reasoning_effort").and_then(|v| v.as_str()) == Some("xhigh")
+            body.pointer("/thinking/effort").and_then(|v| v.as_str()) == Some("max")
         })
     });
     assert!(
         effort_on_wire,
-        "legacy reasoning_effort=xhigh must reach the wire\n{}",
+        "legacy reasoning_effort=xhigh must reach the wire as thinking.effort=max\n{}",
         server.request_log_summary()
     );
 }