docs(scrypt): check in vendor AsyncAPI spec as reference

[TaprootFreak]

Why

Scrypt's WebSocket API is documented behind HTTP Basic Auth at doc.client.scrypt.swiss — credentials are in Vaultwarden, not in any developer onboarding doc. Anyone trying to reason about scrypt.service.ts, scrypt-websocket-connection.ts, or scrypt.adapter.ts today has to know the URL exists and get the password before they can see the schema. That has already cost us once: during the 2026-05-21 BTC/EUR fee investigation, the question "does the venue charge a separate fee or embed it in the spread?" could only be answered by fetching the spec live.

Check the rendered AsyncAPI 3.0 YAML into the repo next to the integration code so the source-of-record schema is reachable via grep, survives credential rotation, and lands in clones used by air-gapped reviewers.

What

• New directory src/integration/exchange/docs/
• scrypt-asyncapi.yaml — verbatim spec download from https://doc.client.scrypt.swiss/v1/client/customer/documentation/docs/public (130 KB, AsyncAPI 3.0.0, Scrypt WebSocket API v1.0.0)
• README.md — source URL, fetch date, refresh procedure, and three non-obvious integration-relevant facts pulled from the spec:
  • Fee on Trade and LastFee on ExecutionReport come back as "0" for our Limit orders; the cost lives in the bid/ask spread
  • AvgPx doc string: "Does not include fees"
  • OrdType: RFQ TradedPx is "all-in price that includes fees" — different semantics, future RFQ integration needs its own pricing logic

Non-changes

• No code changes
• No secrets or credentials in either file (auth headers documented as schemas only, no key values)
• Scope is intentionally just Scrypt; if we want to do the same for Kraken/Binance/MEXC, that's a follow-up

Test plan

• CI green (this PR only touches docs; lint/format/build/tests should be unaffected)
• Verify the spec parses with an AsyncAPI tool of choice (optional)
• Spot-check README.md claims against the YAML