sync: filter peers with head_slot=0 in selectPeer

[zclawz]

Context

From PR #909 review (ch4r10t33r, item #2).

Peers that report head_slot=0 are either freshly connected (no status exchanged yet) or genuinely empty. Routing blocks-by-root requests to them wastes a round-trip and a timeout cycle.

Proposal

Add an optional min_slot parameter to selectPeer() that filters out peers whose last-known head_slot is below the threshold.

Acceptance criteria

• selectPeer accepts an optional minimum slot
• Peers with head_slot < min_slot excluded from candidate set
• Falls back to full set if no peer meets threshold
• Existing callers unchanged

Related: PR #909