refactor(sync): drop _SyncStoreView adapter, make SyncService implement StoreView

[tcoratger]

Summary

_SyncStoreView was a dataclass wrapping a lambda wrapping a getter on
SyncService.store. Three layers of indirection to read two fields.
SyncService can satisfy the StoreView protocol structurally with
two short methods, removing the wrapper without growing the call
surface.

Net: -28 lines, zero behavior change.

What changed

service.py

• Removed the entire _SyncStoreView dataclass.
• Added has_root(root) and head_slot() methods on SyncService
  itself. Each is a one-liner reading self.store.
• Changed the wiring in _initialize_subservices:
  store_view=self instead of _SyncStoreView(_get_store=lambda: self.store).
  The live store reference is observed because backfill calls back
  through self and reads self.store on demand.

backfill_sync.py

• Removed the finalized_slot() method from the StoreView
  Protocol. It was declared on the protocol, implemented on the
  removed wrapper and on FakeStoreView, and never called from
  any production code in backfill_sync.py. Pure protocol bloat.
• Reformatted the surviving Protocol docstring.

test_backfill_sync.py

• Removed finalized_slot() method and the finalized: Slot
  field from FakeStoreView.
• Removed two store_view.finalized = Slot(10) dead writes
  (lines 379 and 411). These had no effect on production code or
  assertions — they were leftover setup from when the protocol
  included finalized_slot.

Why StoreView stays

The protocol earns its 8 lines:

• Breaks a real circular dependency. service.py imports
  BackfillSync from backfill_sync.py. Typing the field as
  SyncService would create a cycle. The Protocol sidesteps this.
• Test seam. FakeStoreView is a 5-line dataclass. Without the
  protocol, tests would need to construct real Store instances
  (with blocks, head, latest_finalized, config, etc.) just to
  exercise backfill.
• Documents the contract. Two methods. A reader of BackfillSync
  sees exactly what it needs from forkchoice without grepping.

Test plan

• ruff check, ruff format --check, ty check all pass
• pytest tests/lean_spec/subspecs/sync -> 156 passed

Out of scope

The next queued items from the sync-layer review:

• make_default_block_processor factory -> method
• SyncService marketing-tone docstring (Reactive/Simple/Resilient/Observable)

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