perf: lazy stream seeding, prepared interventions, cheaper resampling and lgamma

[whilo]

Round 2 of the stack-wide performance work (agent-assisted, reviewed). Fixed-seed outputs bit-identical on all benchmark workloads.

• 2dd8603: keyed random streams are seeded on their first draw, not when created.
• 2476021: conditional resampling reuses its cumulative weights.
• 5b636af: a policy prepares its interventions once instead of per site.
• 373b925: lgamma without sequence allocation.

Clean A/B (3 alternating runs, together with spindel #109): SMC 4.55→3.93 s, MH 5.58→4.57 s, particle Gibbs 51.0→43.6 s. Full foerster suite: 200 tests / 3253 assertions pass (incl. random and benchmark namespaces). Tests added for nested and unseeded streams.