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/src/slash/commands/effort_levels.rs

@@ -5,7 +5,10 @@ use kigi_shell::sampling::types::{ReasoningEffort, ReasoningEffortOption};
 use crate::slash::command::ArgItem;
 
 /// Effort levels in the built-in fallback menu (strongest first). `none`/`minimal`
-/// are still accepted by `ReasoningEffort::from_str` for power users.
+/// are still accepted by `ReasoningEffort::from_str` for power users. `max` is
+/// deliberately absent: it exists only where a model's server menu offers it
+/// (e.g. Kimi K3) — the legacy fallback reproduces the historical rows, and
+/// offering `max` on models that reject it would 400.
 pub(crate) const EFFORT_LEVELS: &[ReasoningEffort] = &[
     ReasoningEffort::Xhigh,
     ReasoningEffort::High,
@@ -20,7 +23,8 @@ pub(crate) fn effort_description(level: ReasoningEffort) -> &'static str {
         ReasoningEffort::Low => "Faster, lighter reasoning",
         ReasoningEffort::Medium => "Balanced reasoning",
         ReasoningEffort::High => "Heavy reasoning",
-        ReasoningEffort::Xhigh => "Maximum reasoning",
+        ReasoningEffort::Xhigh => "Extra-heavy reasoning",
+        ReasoningEffort::Max => "Maximum reasoning",
     }
 }