HWI parity: complete Ledger registered-wallet signing

[trevarj]

Goal

Complete Ledger signtx parity for registered wallets and non-default wallet policies, moving the Ledger matrix entry from [~] to [x].

This follows the remaining partial entry in docs/HWI.md.

Current state

• Standard singlesig signing is covered.
• The parity harness covers one registered native-SegWit 2-of-2 sorted-multisig policy.
• The CLI can reconstruct and register a Ledger multisig policy, retain the returned HMAC, and pass it into the signing interpreter.
• Broader policy shapes and registered-wallet behavior have not been audited or claimed complete.

Scope

• Audit the policy shapes supported by pinned HWI 3.2.0 and the Ledger app used by BHWI.
• Define the exact supported parity set before implementation.
• Extend Ledger policy reconstruction, registration/HMAC handling, and signing context as required.
• Add differential cases for the additional supported policy shapes, including script wrappers and key ordering where applicable.
• Cover already-registered and newly-registered wallet behavior when the emulator supports both paths.
• Return contextual errors for unsupported or ambiguous PSBT policy shapes.

Acceptance criteria

• The supported Ledger wallet-policy set is documented.
• Every claimed policy shape signs successfully through the common, async, and HWI CLI path.
• Added signatures are cryptographically valid and belong to the expected device keys.
• Registration and HMAC handling are emulator-covered.
• hwi-parity-ledger and the unmodified hwi-upstream-ledger final gate pass.
• docs/HWI.md marks Ledger signtx as [x].
• Required protocol, CLI, emulator, and non-emulator checks pass.