fix(node): keep slot_index canonical across side-branch imports and reorgs

[adust09]

Summary

Closes #1201.

get_block_root_by_slot documents slot_index as returning the canonical block root at a slot, but _persist_block wrote put_block_root_by_slot(block.slot, block_root) for every imported block. Because the table is keyed by slot INTEGER PRIMARY KEY with INSERT OR REPLACE, a side-branch import silently overwrote the canonical entry at its slot, and nothing repaired the index when the head moved.

Changes

• SyncService now tracks the head root the database last committed (_persisted_head_root, seeded from get_head_root() at wiring time so a restarted node continues from the persisted chain). Inside the existing persistence batch, _reindex_canonical_slots diffs the old and new head branches down to their fork point: slots on the new canonical branch are (re)written, slots only the old branch filled are deleted, and a block that did not move the head no longer touches the index. The tracker advances only after the batch commits, so a rollback leaves it consistent with disk.
• Database / SQLiteDatabase: new delete_block_root_by_slot(slot) for the vacated-slot case (a reorg onto a branch with an empty slot).
• Testing doubles: MockForkchoiceStore gains an advance_head_on_block knob to simulate side-branch imports; RecordingSyncDatabase gains get_head_root / delete_block_root_by_slot.

A block that does not move the head can never be a head ancestor — on_block only accepts blocks whose parent is already in the store, so every head ancestor is present before import. Skipping the index write for such blocks is therefore always safe (no canonical entry is missed).

Tests

• Side-branch import leaves the slot index untouched while still persisting the head pointer.
• A reorg rewrites the differing slots and deletes entries for slots the new branch leaves empty (and never deletes a refilled slot).
• delete_block_root_by_slot removes only the targeted slot; deleting an absent slot is a no-op.
• Full suite: 2948 passed; just check clean.