feat(distribution-probe): add onProgress callback to probeMany

[ddeboer]

What

probeMany gains an optional onProgress(completed, total) callback, fired once after each probe settles and ending at (total, total).

Why

It lets a caller drive a determinate progress indicator while a large batch runs — the total (distinct probe targets) is known up front, and each settle is a tick. The motivating consumer is the NDE Dataset Register: validating a catalogue with many distributions on one host can take a while (especially with the per-host concurrency cap throttling the burst), and today the UI just hangs. This is the foundation for a progress bar there.

How

• New onProgress?: (completed, total) => void on ProbeManyOptions.
• probeMany wraps the per-probe task so the callback fires after each probe() resolves, counting completions (not result position, since results resolve in input order but tasks settle out of order). The generic mapHostLimited scheduler is untouched.

Notes

• Order reflects completion, not input order; fires total times; never called for an empty batch.
• A throwing callback rejects the batch (documented), so it is meant to be cheap and side-effect-only.
• Additive, optional — non-breaking.

Tests

Added: onProgress ticks once per probe ending at (total, total); not called for an empty batch. Full @lde/distribution-probe suite green (91 tests), typecheck clean.