Add provider adapters and the first one

[rokoss21]

ROADMAP item 3.

target_provider_id was the literal string "unknown-provider", there was no adapters directory, and the cross-vendor promise — the reason Canonical JSON exists at all — had no implementation and nothing testing it.

The obligations, as a trait rather than as prose

crates/fct-adapter says what an adapter owes:

1. Message order and roles survive (§18.1.2) — no reordering, merging or dropping.
2. Tool identity survives. Canonical names are Interface.fn (§16.2.6) — what policy matches on. Providers allow only [A-Za-z0-9_-], so the adapter must mangle, and the mangling must reverse.
3. Schemas cross unchanged (Appendix D).
4. The target is attributed — provider_id lands in metadata.target_provider_id.

No I/O in the crate. Translation is pure, which is what makes "canonical semantics survive this" falsifiable.

OpenAiCompatAdapter

Covers OpenAI and everything on the same wire format: DeepSeek, Together, vLLM, llama.cpp's server, LM Studio.

Canonical	Provider
WeatherAPI.get_current	WeatherAPI__get_current

Two canonical names that would share a provider name are an error. A returned call mapping to no tool this request exposed is an error. Neither is guessed at — the guard decides on the canonical name, so a wrong mapping means a policy answered by the wrong rule.

End to end

facet-fct agent --input contract.facet --script recorded.json \
  --provider openai-compat --model deepseek-chat --exec

The recording holds raw provider responses, so the translation is part of the run rather than asserted about in isolation:

{"final_text":"It is raining in Minsk, 11C.","target_provider_id":"openai-compat.v1",
 "guard_events":4,"stop":"completed","turns":2}

The provider called WeatherAPI__get_current; the artifact records WeatherAPI.get_current.

Verification

497 tests (6 adapter unit tests over recorded traffic, 1 CLI test through the whole loop), smoke, matrix, doc-examples, coverage gate — all green.

Not done, and written down

• No HTTP. Carrying bytes is the host's job, and it belongs behind a feature flag so neither CI nor a compile-only workflow needs a network stack.
• One adapter proves little. Anthropic's format differs enough — system is a top-level field, tool results have their own content block — to be the first real test of whether these obligations are the right ones.
• target_provider_id deliberately does not enter the provenance chain: Appendix F.4 enumerates the seed and the target is not in it.

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