feat: add dashd integration tests for SPV sync

[xdustinface]

Adds comprehensive integration tests that verify SPV sync against a real dashd instance with pre-generated regtest blockchain: dashpay/regtest-blockchain

Includes:

• CI infrastructure: contrib/setup-dashd.py for cross-platform dashd/test-data setup, GitHub Actions caching and log retention on test failure
• Shared test utilities in dash-spv/src/test_utils/: DashCoreNode (dashd process management, RPC), DashdTestContext (common setup), filesystem helpers
• FFI test utilities in dash-spv-ffi/src/test_utils/: CallbackTracker (callback verification), FFITestContext (FFI client wrapper lifecycle management)
• Thread-local logging support for parallel test isolation
• SPV tests: basic sync, empty wallet, multi-wallet, restart consistency, restart with fresh wallet, multiple restarts, random restarts, peer disconnection (exclusive and non-exclusive mode), incremental transactions (single block, across blocks)
• FFI tests: wallet sync, incremental sync, restart consistency, all-callbacks verification, post-sync transaction and disconnect callbacks

Based on:

• feat: add cycle to SyncEvent::SyncComplete #459

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch feat/dashd-integration-tests

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