FanoutTree: join candidate scoring modes (formation optimization)

[marcus-pousette]

Parent: #585

Goal

Use the new formation metrics/score to iteratively improve joiner-side parent selection (tree stays shallow, balanced, low stretch) without requiring global membership knowledge.

Work

• Define a scoring function over tracker candidates (level, freeSlots, bidPerByte, already-connected, recent failures).
• Add an explicit scoring mode/weights in FanoutTree join options (defaults stay simple).
• In sims, compare formationScore + stretch + attachP95 across modes on fixed seeds (ci-small + scale-5k).

Acceptance

• One new scoring mode shows measurable improvement (lower formationScore and/or depthP95) on scale-5k with no regression in delivery/overhead.