Phase C1: per-tx fee + fee-priority transaction selection for over-cap blocks

[LiranCohen]

For an over-cap block, compute each ION tx's fee = sum(input prevout values) - sum(outputs) (via B1 per input) and its writer (B2), then select per the reference TransactionSelector (one tx per writer, then transactionFeePaid desc, txnum tiebreak) up to the 300-tx / 600000-op budget. Replaces the txnum-order selection in internal/process/processor.go processAnchors.

Plan: docs/plans/2026-06-04-002-feat-bitcoind-free-ion-compliance-plan.md (Phase C1). Latent — no mainnet block has approached the caps; depends on B1, B2.