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-sampler/src/kimi_compat.rs

@@ -292,8 +292,9 @@ mod tests {
             json!({ "type": "enabled", "effort": "high" })
         );
 
-        // Canonical `xhigh` is spelled `max` on the Kimi wire (the K3
-        // valid_efforts vocabulary is low/high/max).
+        // Legacy canonical `xhigh` (pre-Max configs/sessions) is spelled
+        // `max` on the Kimi wire (the K3 valid_efforts vocabulary is
+        // low/high/max — there is no `xhigh` there).
         let mut body = json!({ "model": "k3", "reasoning_effort": "xhigh" });
         adapt_chat_completions_body(&mut body);
         assert_eq!(
@@ -301,6 +302,15 @@ mod tests {
             json!({ "type": "enabled", "effort": "max" })
         );
 
+        // Canonical `max` (what the K3 menu token parses to since the
+        // ReasoningEffort::Max split) passes through unchanged.
+        let mut body = json!({ "model": "k3", "reasoning_effort": "max" });
+        adapt_chat_completions_body(&mut body);
+        assert_eq!(
+            body["thinking"],
+            json!({ "type": "enabled", "effort": "max" })
+        );
+
         // kimi.py:218: "off" (our ReasoningEffort::None) → disabled, and no
         // effort key (a disabled+effort combination would be contradictory).
         let mut body = json!({ "reasoning_effort": "none" });