refactor(sync): converge duplicated gap-fetch arithmetic in head sync

[tcoratger]

Summary

The same start/floor/size computation for range-fetching a contiguous gap of blocks above the chain head was duplicated in two places under src/lean_spec/node/sync/:

• Head sync, when caching an orphan gossip block whose parent is missing.
• Backfill sync, when received blocks still have a missing parent far above the highest known slot.

Both computed the gap floor as head + 1, derived the gap size, logged, and dispatched to a range fetch when the size was positive.

This change factors that logic into a single fill_gap_above_head method on the backfill syncer that returns whether a range fetch was issued. Both call sites route through it:

• Head sync falls back to parent-by-root recursion when the gap is empty (single-slot gap above head, or alt-fork gossip at or below head).
• Backfill keeps its unconditional root recursion afterward.

Behavior and bounds are preserved exactly. The now-unused Slot and Uint64 imports were dropped from the head sync module.

Testing

• uv run pytest tests/node/sync/test_head_sync.py tests/node/sync/test_backfill_sync.py — all pass.
• just check — clean (lint, format, typecheck, spell, mdformat, lock).

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