feat(rfq): validate final settlement PSETs

[tvolk131]

Summary

• add a provider-side validator for a complete blinded, taker-signed Liquid PSET before the irreversible signing transition
• derive authority from the persisted firm quote, authenticated reservation, durable wallet targets, authoritative unspent prevouts, and provider-controlled output recovery
• locate the RFQ leg inside a larger transaction through an injective symbolic layout without assuming fixed input or output indexes
• verify canonical bounded PSET structure, finalized taker P2TR SIGHASH_ALL signatures, unsigned provider inputs, confidential disclosures and proofs, whole-transaction balance, provider receive/change recovery, and one exact policy-asset fee output
• project the missing provider witnesses to derive regular and discounted size metrics, then enforce the quote's frozen fee and maximum-weight policy
• make an opaque non-cloneable ValidatedSigningIntent the only production capability that can cross commit-before-sign
• recognize exact committed and signed retries from durable state before live chain or wallet calls
• document the initial one-provider/P2TR profile and the unavoidable availability risk from a taker conflict-spending its own input after the final chain observation

Why

PR #29 constructs and durably reserves an inventory-aware FirmQuote, but the provider still needed a concrete authority boundary for the complete transaction returned by the taker. This PR closes that boundary: detached caller assertions cannot choose the signing payload or fee facts, and the wallet/HSM can only be invoked after exact validated bytes have crossed the durable point of no return.

Validation itself remains reversible. The eventual service should validate and commit synchronously; the commit atomically rechecks provider identity, authenticated owner, quote commitment, exclusive deadline, fee policy, and every reserved allocation before it returns a durable signing job.

Safety properties

• raw bytes are size-bounded before PSET decoding and only one strict canonical representation is accepted
• every input is bound to an ordered authoritative unspent prevout from the provider's configured chain
• every non-provider input must already carry a valid tree-less P2TR key-path explicit-SIGHASH_ALL signature over the complete transaction and all prevouts
• every provider input must match its persisted quote/inventory/signing target and remain unsigned
• issuance, peg-ins, nonfinal sequences, locktimes, mutable PSETs, unsupported metadata, duplicate inputs, and aliased role mappings fail closed
• ordinary outputs require confidential commitments plus valid disclosure, range, and surjection proofs; the complete transaction must balance
• provider payment and change outputs must resolve through durable wallet locators to the expected spend keys and confidential openings
• the fee policy uses the projected final provider witnesses rather than an underweight pre-sign transaction
• dropping a validated intent performs no mutation; cancellation or expiry can still win before commit
• after commitment, only the exact canonical payload replays, including after restart or after its inputs have been spent

Deliberate V1 scope

The validator supports one interactive RFQ provider plus already-finalized tree-less P2TR wallet inputs. It does not yet execute Simplicity covenant witnesses or coordinate a second interactive RFQ signer. Those require a later authenticated venue/script-verification seam.

Concrete wallet/RPC/HSM adapters, signer/finalizer orchestration, authenticated networking, production pricing, relay, and chain reconciliation remain follow-up work.

Stack

This is intentionally stacked on #29 and targets codex/rfq-quote-engine. Once #29 merges, this PR can be retargeted to master.

Validation

• nix develop .#default --command just ci-checks
  • generated-contract verification and formatting passed
  • strict workspace Clippy with warnings denied passed
  • 312 workspace tests passed; 6 intentionally skipped
  • all workspace documentation tests passed
  • WASM build check passed
• all 79 deadcat-rfq-provider tests passed
• 12 focused settlement tests cover the valid flow, real fee-policy rejection, cancellation/deadline races, cross-provider binding, committed/signed replay, output recovery, layout aliasing, and more than 30 fail-closed PSET/authority mutations

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