Check Ledger firmware version before attempting to sign

[Kingsman-99]

Description

adapters/ledger.ts opens a transport and signs without verifying the Ledger device's firmware version. Outdated firmware versions lack support for newer Stellar transaction features, causing silent signing failures.

Acceptance Criteria

• Before signing, the adapter queries the device firmware version via the transport
• Versions below MIN_LEDGER_FIRMWARE (export this constant) reject with a LedgerFirmwareTooOldError listing the required version
• Compatible firmware versions proceed with the existing signing flow
• Unit tests pass

Context

• Target file: src/adapters/ledger.ts
• The firmware check should be skipped in test environments via a constructor flag