Follow-up to #1838 probes.base: balance IntentProbe pruning across intents. It balances the cap across intents within one probe; this tracks the broader ask: stub selection that is reproducible and shared across probes.

Today Probe._prune_data samples with the global unseeded random and IntentService returns unordered sets, so run.seed has no effect and probes over the same intent prune to different stubs.

Wanted:

• Stable per-intent stub ordering plus an RNG seeded from run.seed (prerequisite).
• The same surviving stub subset across every probe using an intent in a run.
• Same run.seed reproduces the selection.

Related:

• Confidence Interval consistency #1646
• probes.base: balance IntentProbe pruning across intents #1838
• support continuation / resume of runs #141