Fix correctness issues in the Unix socket IPC layer and add comprehensive
test coverage for the CrowIPC package.

Fixes:
- Handle partial writes in SocketServer and SocketClient (write loop)
- Add 30s read timeout to SocketClient via SO_RCVTIMEO
- Add 1MB response size limit to SocketClient matching server's request limit
- Fix 3 RPC handlers returning errors in result dict instead of proper
  JSON-RPC error responses (get-session, close-terminal)

Improvements:
- Add RPCErrorCoded protocol for typed error codes in CommandRouter
- Conform RPCError to RPCErrorCoded with correct code mapping
- Add missing doubleValue accessor, internalError code, Hashable on JSONValue
- Add doc comments to all public types
- Add new SocketError cases (writeFailed, readFailed, timeout, responseTooLarge)

Tests (32 passing):
- ProtocolTests: JSONValue round-trips, accessors, factories, error codes
- CommandRouterTests: routing, unknown methods, error propagation
- SocketRoundTripTests: real socket round-trips, large payloads, timeouts,
  oversized requests, permissions, invalid JSON

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>