geoprobe: probe newly discovered targets immediately

[ben-dz]

Resolves: #2924

Summary of Changes

• Newly discovered onchain targets are probed immediately upon discovery instead of waiting up to 5 minutes for the next measurement ticker
• Extracted sendCompositeOffsets from runMeasurementCycle so the offset composition/signing/sending logic can be reused for immediate probes
• Added Pinger.MeasureOne to measure a single target using the same warmup+measurement probe pattern as MeasureAll

Diff Breakdown

Category	Files	Lines (+/-)	Net
Core logic	2	+87 / -6	+81

All net-new logic; no tests, config, or docs changes.

Key files (click to expand)

• controlplane/telemetry/internal/geoprobe/pinger.go — new MeasureOne method for single-target measurement
• controlplane/telemetry/cmd/geoprobe-agent/main.go — immediate probe on target discovery; extracted sendCompositeOffsets helper

Testing Verification

• Existing geoprobe unit tests pass (including pinger concurrency tests)
• Build verified in dev container