geolocation: add result destination for LocationOffsets

[ben-dz]

Summary of Changes

• Add optional result_destination string field to GeolocationUser so measurement results (LocationOffsets) can be sent to an alternate endpoint instead of the target IP — critical for ICMP targets that aren't running geolocation software, and useful for centralizing result collection
• Supports both IP and domain destinations in host:port format (e.g., 185.199.108.1:9000 or results.example.com:9000); empty string = not set
• New SetResultDestination onchain instruction validates the destination (public IP or valid domain + port), updates the field, and bumps target_update_count on all referenced probes so agents re-discover the change
• CLI: dz-cli geolocation user set-result-destination --user <code> --destination <host:port> (or --clear), with client-side validation before transaction submission
• Go SDK deserializes the new field with backwards compatibility (old accounts default to "")
• Known limitation: If a user with targets that use more than 25 unique probes tries to change their destination IP, the command will fail. This is unlikely in early days and will be addressed by batching in the future.

RFC: rfcs/rrfc16-geolocation-verification.md

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	8	+427 / -35	+392
Scaffolding	11	+314 / -0	+314
Tests	2	+494 / -4	+490
Docs	1	+1 / -0	+1

~40% of net additions are tests; core logic includes the onchain processor with domain/IP validation, Rust SDK command, and Go SDK deserialization.

Key files (click to expand)

• smartcontract/cli/src/geolocation/user/set_result_destination.rs — CLI command with clap args, client-side destination validation (IP + domain), mock tests
• smartcontract/programs/doublezero-geolocation/src/processors/geolocation_user/set_result_destination.rs — onchain processor: validates signer, parses host:port, checks IP routability or domain format, bumps probe target_update_count
• smartcontract/sdk/rs/src/geolocation/geolocation_user/set_result_destination.rs — Rust SDK command that builds the transaction with user PDA + probe account metas
• smartcontract/programs/doublezero-geolocation/src/state/geolocation_user.rs — new result_destination: String field with BorshDeserializeIncremental, backwards-compat via #[incremental(default)]
• sdk/geolocation/go/state.go — Go SDK struct + serialize/deserialize with try-decode-default for old accounts
• smartcontract/programs/doublezero-geolocation/src/instructions.rs — SetResultDestination variant added to instruction enum

Testing Verification

• 7 integration tests: success with IP destination, success with domain destination, clear, unauthorized signer, invalid IP, invalid format (missing port), no-targets user
• Backwards-compat unit tests in both Rust and Go: serialize with new field, truncate trailing bytes, verify default
• Onchain processor unit tests for domain validation (valid domains, edge cases)
• CLI validation tests covering 9 invalid destination cases (private IPs, bad ports, malformed domains)
• Instruction roundtrip serialization tests
• All existing tests pass: 83 geolocation program tests, 165 SDK tests, 288 CLI tests, 49 Go SDK tests