Add legacy P2SH multisig support

[Jacksper13]

What changed

Adds focused support for legacy sh(sortedmulti(...)) multisig accounts:

• imports and serializes legacy P2SH descriptors, including crypto-output
• preserves per-cosigner BIP-45 derivation prefixes
• reconstructs legacy account descriptors from PSBT inputs and requires non_witness_utxo
• routes registered legacy inputs and outputs through the shared registered-multisig path and script validation
• enforces the 15-key legacy script limit
• adds P2SH-specific regression coverage

Dependencies

This branch is temporarily stacked on:

• Preserve multisig derivation prefixes #139, which preserves fixed derivation prefixes during multisig import
• Generalize registered multisig path validation #140, which provides the shared registered-multisig path resolver and output classifier

Once those merge, this branch can be rebased onto main without changing its P2SH behavior.

Scope

The general multisig PSBT hardening was moved to and merged through #134. This PR now contains only the remaining bare-P2SH compatibility layer on top of the shared path handling.

[nicoburniske]

this adds a lot of legacy p2sh/bip-45 and general multisig code that isn't needed for the security fix, which makes it much harder to review. i think the fix should stay focused on validating against the stored MultiSigDetails, with legacy support handled separately.

#134

[mjg-foundation]

this work is required for signing unchained and future casa multisig PSBTs, needs to ship alongside those exports in 1.4.0. Make sure to tag ngwallet after merging, and make a PR to KeyOS to bump the ngwallet version to that tag (or latest).

[mjg-foundation]

Review found 1 urgent and 1 high issue in the legacy P2SH compatibility layer.

[Jacksper13]

@mjg-foundation Follow-up is now split by the code that owns each invariant: #139 preserves and round-trips fixed crypto-output derivation prefixes, #140 provides the shared registered-multisig path/script validation, and #131 remains focused on the legacy P2SH wrapper. The remaining KeyOS P2SH export mapping is tracked internally for the 1.5.0 integration; P2SH is intentionally not being exposed in KeyOS 1.4.0. I have left the review threads open for your verification.

[mjg-foundation]

Re-reviewed the new commits — no new issues.