docs: add private market-maker guidance (COW-1262)

[jean-neiverth]

Summary

• Adds a "Private market makers" subsection under "4. Build a route" in the sub-solver integration guide
• Covers the trust gap when holding inventory in the Trampoline (missing funding transfer risk), the recommended external-contract pattern with transferFrom, and the claimToken fallback
• Adds a pre-launch checklist item for MM inventory placement

Companion ADR: bleu/byos-contracts#39

Test plan

• The recommended pattern is consistent with ADR-0011 in byos-contracts
• Cross-references to interactionsHash, claimToken, and minBuyAmount are accurate
• The section reads naturally between "Build a route" and "Sign the proposal"
• All field names use quoteBuyAmount (not maxBuyAmount or quotedBuyAmount)

[linear-code]

COW-1262