v0.4.19-zr2

Add disk-persisted TreeState cache to reduce validator RPC load

Implements caching for GetTreeState responses with the following approach:

Caching strategy:
- Configurable window size (default: 525,600 blocks / ~1 year)
- Blocks older than the window are NOT cached (always fetch from zcashd)
- Deep blocks (≥100 confirmations): cached indefinitely within window
- Recent blocks (<100 confirmations): 2-second TTL to handle reorgs

Storage:
- Two flat files: treestates.idx (index) and treestates.dat (data)
- Sparse storage - only caches heights that are actually requested
- Checksums for corruption detection; corrupted cache auto-clears

Configuration:
- New CLI flag: --treestate-cache-window (default: 525600, 0 to disable)
- Cache integrates with BlockIngestor for reorg notifications

Also fixes go vet warning in GetLightdInfo by using proto.Clone()
instead of direct struct copy for protobuf messages containing mutex.

v0.4.19-zr1

Use Alpine-based Dockerfile with Go 1.25, remove zcash workflow depen…