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/src/agent/models_fetch.rs

@@ -338,7 +338,7 @@ fn think_efforts_to_options(
         .collect()
 }
 
-fn platform_wire_model_to_entry(
+pub(crate) fn platform_wire_model_to_entry(
     platform: kigi_models::PlatformId,
     wire: kigi_models::WireModel,
     base_url: &str,
@@ -703,7 +703,9 @@ mod tests {
             "https://api.kimi.com/coding/v1",
         );
         assert!(entry.supports_reasoning_effort);
-        assert_eq!(entry.reasoning_effort, Some(ReasoningEffort::Xhigh));
+        // The wire token "max" is canonical Max since the Xhigh/Max split;
+        // kimi_compat still spells it "max" on the inference wire.
+        assert_eq!(entry.reasoning_effort, Some(ReasoningEffort::Max));
         let ids: Vec<&str> = entry
             .reasoning_efforts
             .iter()
@@ -723,7 +725,7 @@ mod tests {
             [
                 ReasoningEffort::Low,
                 ReasoningEffort::High,
-                ReasoningEffort::Xhigh
+                ReasoningEffort::Max
             ],
         );
         let max = entry