telemetry/geoprobe: add version and target IP to LocationOffset wire format

[ben-dz]

Summary of Changes

• Adds Version (uint8) and TargetIP ([4]byte) fields to the LocationOffset struct, bumping the wire format to v1 (174 bytes, up from 169)
• Version is validated on unmarshal — unknown versions are rejected with a clear error
• Target IP captures the IPv4 address of the TWAMP measurement target, propagated through the reference chain
• Updates all serialization paths (Marshal, Unmarshal, GetSigningBytes) and the raw-byte ParseOffsetInfo parser
• Updates RFC16 to match the implementation

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	5	+65 / -5	+60
Tests	3	+89 / -7	+82
Docs	1	+2 / -1	+1

~58% of new lines are tests.

Key files (click to expand)

• controlplane/telemetry/internal/geoprobe/offset.go — new Version/TargetIP fields, IPToTargetIP/FormatTargetIP helpers, updated Marshal/Unmarshal/GetSigningBytes with version check
• controlplane/telemetry/internal/geoprobe/offset_test.go — updated all fixtures for new fields, added TestLocationOffset_UnsupportedVersion
• controlplane/telemetry/internal/geoprobe/signer_test.go — updated all signing/verification test fixtures
• tools/twamp/pkg/signed/packet.go — adjusted LocationOffsetSize (169→174), shifted byte offsets in ParseOffsetInfo, added TargetIP to OffsetInfo
• tools/twamp/pkg/signed/packet_test.go — updated byte offsets in test blob construction, added TargetIP assertion
• controlplane/telemetry/cmd/geoprobe-target/main.go — surface TargetIP in JSON and text output
• controlplane/telemetry/cmd/geoprobe-agent/main.go — populate Version and TargetIP when constructing offsets
• controlplane/telemetry/internal/geoprobe/publisher.go — populate Version and TargetIP in DZD publisher path

Testing Verification

• All existing offset Marshal/Unmarshal, signing, and chain verification tests updated and passing with new fields
• New TestLocationOffset_UnsupportedVersion test validates that corrupted version byte is rejected
• TestLocationOffset_Size updated to assert the new 174-byte size
• TestParseOffsetInfo validates correct extraction of TargetIP from raw bytes at the new offsets