feat(sdk-coin-dot): add MPCv2 signed hot recovery support

[ralph-bitgo]

What

• Adds isMpcv2SigningMaterial() and addRecoverySignature() private methods to Dot (modules/sdk-coin-dot/src/dot.ts), and updates recover() to detect the keycard format once (MPCv1 JSON vs MPCv2 CBOR) and dispatch signing accordingly.
• Reuses the shared EdDSA MPCv2 recovery helpers already extracted into @bitgo/sdk-core (getEddsaSigningMaterial, signEddsaMpcV2RecoveryTx) rather than duplicating logic — same pattern as sdk-coin-sol (WCI-398) and abstract-substrate (WCI-1276/WCI-1239).
• The MPCv2 path runs the MPS DSG protocol locally and hands the raw 64-byte Ed25519 signature to the transaction builder without manually prepending the 0x00 Substrate discriminant, because Transaction#constructSignedPayload already prepends it unconditionally for every signature (the same mechanism the pre-existing MPCv1 path relies on). Manually prepending it, as literally described in the ticket and mirrored from the already-merged abstract-substrate code, double-prefixes and corrupts the signature — verified empirically by round-tripping a signed transaction through decode.
• Adds a describe('Recover Transactions (MPCv2):') test block covering: MPCv2 signed recovery returning { serializedTx, scanIndex }, MPCv1 regression (still uses getTSSSignature), mismatched bitgoKey vs keycard commonKeyChain throwing, and a cryptographic verification (via nacl.sign.detached.verify against the transaction's signable payload and derived pubkey) that the extrinsic signature is exactly 64 bytes with the discriminant applied once — not just a length check, which would not catch the double-prefix corruption class.
• recoverConsolidations() MPCv2 support is intentionally out of scope here — tracked separately in WCI-1236.

Why

Wallets are migrating EdDSA MPC from the Zengo implementation (MPCv1) to Silence Labs (MPCv2). DOT's recover() only handled MPCv1 keycards, so self-custody hot recovery would fail for any wallet backed by an MPCv2 keycard. This closes that gap for DOT, matching the detection + signing path already shipped for SOL (WCI-398) and the shared helpers extracted for Substrate coins (WCI-1276/WCI-1239).

Test plan

• yarn unit-test in modules/sdk-coin-dot — 205 passing, 0 failing (includes the 5 new MPCv2 tests)
• yarn build:cjs (tsc --build) — compiles cleanly
• yarn eslint --quiet src/dot.ts test/unit/dot.ts — no errors
• Manually confirmed the "0x00 discriminant" test fails if the double-prefix bug is reintroduced (temporarily re-added the manual prefix, observed the cryptographic verification correctly fail, then reverted)

Ticket: WCI-1227

[linear-code]

WCI-1227