fix: drop sampling knobs for Claude 4.7+ and clamp xhigh on 4.6

[CMGS]

Problem

claude_engine::build_body kept temperature / top_p (and top_k via raw extras) unless the gateway had itself mapped a reasoning request into thinking. That gate models Claude 4.6, where sampling parameters are accepted until thinking is on. Every later generation — Opus 4.7/4.8, Sonnet 5, Opus 5, Fable 5/5.1 — rejects the three parameters outright with a 400, and OpenAI-compatible clients send temperature by default, so a plain chat request to claude-opus-5 failed upstream.

map_thinking also passed the client's effort through verbatim, so reasoning_effort: xhigh (introduced with Opus 4.7) or an OpenRouter-style budget ≥ 20480 reached Claude 4.6 as output_config.effort: xhigh, which 4.6 does not accept. The live matrix already knew (its 4.6 tiers omit xhigh), the engine did not.

Change

• Sampling knobs are dropped whenever the model's dialect is AdaptiveSummarized (4.7+ and the 5 family), and still only after a thinking mapping on Adaptive (4.6); the ≤4.5 budget dialect keeps them.
• map_thinking clamps xhigh → high on the Adaptive dialect only.
• docs/api.md states both rules; scripts/live-matrix gains a claude-opus-5 row with effort tiers including xhigh, a temperature-carrying chat call and a native adaptive+summarized stream, so the dialect every current Claude id resolves to has live coverage (none existed).

Evidence

• Unit: sampling_knobs_follow_the_model_generation (4.5 and 4.6 keep the knobs, claude-opus-5 and claude-fable-5-1 drop all three) and xhigh_clamps_to_high_on_claude_4_6_only (effort and budget forms; 4.7+ keeps xhigh). Two integration tests that asserted the old passthrough on claude-fable-5 now assert the drop.
• cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test (workspace): green. python3 -m py_compile on the live matrix: clean. The new live cases need vendor keys and were not run here.

Hot path: one anthropic_thinking_dialect string scan per request, which the reasoning branch already paid; no new allocation.