geolocation: onchain portion of alternate offset-destinations

[ben-dz]

Summary of Changes

• Add a result_destination field to the GeolocationUser onchain account, allowing users to specify an IP:port or domain:port where geoprobe results should be sent
• Add a new SetResultDestination instruction that validates the destination (public IP or valid domain + port), updates the user account, and bumps target_update_count on all referenced probe accounts so they pick up the change
• Use BorshDeserializeIncremental for backward-compatible deserialization of existing accounts that lack the new field

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	6	+292 / -26	+266
Scaffolding	4	+16 / -1	+15
Tests	2	+445 / -2	+443

~60% tests, ~37% core logic — well-covered new instruction.

Key files (click to expand)

• smartcontract/programs/doublezero-geolocation/tests/geolocation_user_test.rs — integration tests for set/clear/unauthorized/invalid-IP/missing-probe/domain scenarios
• smartcontract/programs/doublezero-geolocation/src/processors/geolocation_user/set_result_destination.rs — new processor: validates destination, updates user, bumps probe target_update_count
• smartcontract/programs/doublezero-geolocation/src/state/geolocation_user.rs — add result_destination field with incremental deserialization, refactor TryFrom impls
• smartcontract/programs/doublezero-geolocation/src/instructions.rs — register SetResultDestination variant and args
• smartcontract/programs/doublezero-geolocation/src/entrypoint.rs — route new instruction to processor
• smartcontract/cli/src/geolocation/user/get.rs — display result_destination in CLI get output
• smartcontract/programs/doublezero-geolocation/src/error.rs — add TooManyReferencedProbes error variant

Testing Verification

• Unit tests for destination validation (IP:port, domain:port, private IP rejection, missing port, invalid port, single-label domain, hyphen/underscore edge cases)
• Backward-compatibility unit test: deserializing old account data without the new field defaults to empty string
• Integration tests via BanksClient: successful set, clear (empty string), unauthorized signer rejection, invalid IP rejection, missing probe account rejection, domain destination