Add IGRA exit unsigned transaction CLI

[reshmem]

Summary

• add cast igra build-exit to build kaspawallet-compatible unsigned IGRA exit PST hex plus JSON manifest
• add cast igra verify-exit to verify unsigned/signed artifacts, payload nonce, txid prefix, outputs, locking script, and multisig completion
• derive per-input multisig xpubs exactly as kaspawallet expects and canonicalize signer order by sorted master xpubs

Validation

• cargo test -p foundry-common igra_exit
• cargo check -p cast
• built official kaspawallet from /Users/user/Source/igra/kaspad
• created three temporary 2-of-3 multisig kaspawallet key files
• built an unsigned IGRA exit PST with txid prefix 00 and verified it
• signed with official kaspawallet: signer1 partial, signer2 partial, signer3 partial, signer1+2 fully signed, signer1+3 fully signed
• parsed the fully signed PST with official kaspawallet parse

Note: global cargo fmt --check is currently blocked by pre-existing unrelated formatting diffs; rustfmt --edition 2024 --check crates/common/src/igra_exit.rs crates/cast/src/cmd/igra.rs passes.