geolocation: add geoprobe-target-sender CLI tool

[ben-dz]

Summary

• Add geoprobe-target-sender CLI tool that sends signed TWAMP probes to a GeoProbe and verifies signed replies, implementing the inbound probing flow for geolocation verification (RFC16 Phase 1.5 goal 3)
• Supports configurable probe count, interval, timeout, and both text/JSON output formats
• Includes Ed25519 signature verification for both probe echo and reflector reply

Closes #3110

RFC Reference

rfcs/rfc16-geolocation-verification.md — "Example Target Software for Target Probe Senders" section

Testing Verification

• Unit tests cover pubkey parsing (valid + invalid inputs), RTT formatting, JSON serialization (success, timeout, omitempty), and Ed25519 signer integration with Solana keypairs
• Build, lint, and tests all pass clean in the dev container