feat(client): implement FastStart for early connection and background resolver discovery

[taskkillstar]

Summary

When configured with large resolver lists (e.g. 50–100+ endpoints), the client previously blocked session initialization and local proxy (SOCKS5/DNS) startup until 100% of all resolvers were synchronously probed for MTU.

This PR introduces FastStart:

• When \RECHECK_INACTIVE_SERVERS_ENABLED = true\ (default), the initial MTU test completes early as soon as a minimal working set of resolvers (\min(TotalResolvers, max(1, RX_TX_WORKERS))) is validated.
• The client connects and opens local proxy listeners immediately (~1–2s startup time).
• The remaining unscanned resolvers remain in the pool as \inactive, where the existing background
  unResolverHealthLoop\ discovers and reactivates them asynchronously without blocking user traffic.
• When \RECHECK_INACTIVE_SERVERS_ENABLED = false, the legacy behavior is preserved (full 100% upfront scan).

Key Changes

• **\internal/client/mtu.go**: Added early-exit condition to \RunInitialMTUTests\ bounded by \ argetValid, capped worker count during fast start, and added completion logs.
• **\internal/client/fast_start_test.go**: Added comprehensive unit test suite (\TestRunInitialMTUTests_FastStartExitsEarly, \TestRunInitialMTUTests_FullScanWhenRecheckDisabled, \TestRunInitialMTUTests_FallbackWhenFewerResolversWork).
• **\client_config.toml.simple\ & \README.MD**: Documented FastStart behavior under \RECHECK_INACTIVE_SERVERS_ENABLED.
• Exported helper methods in \internal/config\ and \internal/udpserver\ (\DefaultClientConfig, \DefaultServerConfig, \Server.HandlePacket).

Verification

• \go test -v ./internal/client -run TestRunInitialMTUTests\ -> PASS
• \go test ./...\ -> PASS across all packages
• \go build ./cmd/client && go build ./cmd/server\ -> PASS (clean build)

[taskkillstar]

Minimizes startup time (no need to scan the entire resolvers list), allows to have several hundreds of resolvers in the list without sacrificing connection time.

Requested by FR #179