feat: Add ProximityCache introspection support (Phase 3)

- Add ProximityCacheInfo struct for proximity cache introspection
- Add ContractCacheEntry for tracking cached contracts
- Add NeighborCacheInfo for neighbor cache tracking
- Add ProximityStats for proximity propagation statistics
- Update QueryResponse enum to include ProximityCache variant
- Bump version to 0.1.19

Part of Phase 3 implementation for issue #1848

Author: sanity

rust/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "freenet-stdlib"
-version = "0.1.18"
+version = "0.1.19"
 edition = "2021"
 rust-version = "1.71.1"
 publish = true