Split cache.rs and config.rs into directory modules, create utils/

Module reorganization for three large single-file modules:

cache.rs (792 lines) split into cache/ directory:
- cache/mod.rs: CacheError, CacheStats, re-exports
- cache/entry.rs: CacheEntry with TTL/LRU tracking
- cache/coord_cache.rs: CoordCache (address-to-coordinate mappings)
- cache/route_cache.rs: RouteCache + CachedCoords (discovery routes)
- Added 22 new tests filling coverage gaps across all submodules

config.rs (1318 lines) split into config/ directory:
- config/mod.rs: ConfigError, IdentityConfig, Config struct with file
  loading/merge logic, all 24 integration tests
- config/node.rs: NodeConfig + 9 subsection structs (Limits, RateLimit,
  Retry, Cache, Discovery, Tree, Bloom, Session, Buffers)
- config/transport.rs: TransportInstances<T>, TransportsConfig, UdpConfig
- config/peer.rs: ConnectPolicy, PeerAddress, PeerConfig

DnsConfig and TunConfig moved to upper/config.rs to co-locate with the
upper layer components they configure (TUN interface, DNS responder).

index.rs moved to utils/index.rs as cross-cutting infrastructure that
serves both node and peer layers.

src/node/tests/mod.rs

@@ -1,5 +1,5 @@
 use super::*;
-use crate::index::SessionIndex;
+use crate::utils::index::SessionIndex;
 use crate::transport::{packet_channel, LinkDirection, TransportAddr};
 use crate::PeerIdentity;
 use std::time::Duration;