refactor(sync): collapse SyncState to enum plus single non-trivial property

[tcoratger]

Summary

SyncState was 163 lines for a three-state enum. Most of the surface
was trivial state == SyncState.X wrappers and a state-machine helper
with one caller. Collapses to the enum plus a single non-trivial
property, with the transition logic inlined where it actually runs.

Net: -225 lines across 4 files, zero behavior change.

What changed

states.py — 163 -> 32 lines

Removed:

• is_idle, is_syncing, is_synced properties — each wrapped a
  one-line == comparison. Zero clarity gain, extra API surface.
• can_transition_to(target) method — used at exactly one call site.
• Module-level _VALID_TRANSITIONS dict — only fed the helper above.
• Heavy per-state prose docstrings ("The Lifecycle", "Transitions",
  etc.).

Kept:

• The enum itself.
• accepts_gossip — the only property that does real work
  (self in {SYNCING, SYNCED}), used at 3 call sites.

service.py — small net reduction

Removed SyncService.is_syncing and is_synced public properties.
Grep confirmed zero callers outside the file itself. Anyone needing
them can write service.state == SyncState.X.

Updated 3 call sites that used the removed enum properties:

• self._state.is_syncing -> self._state == SyncState.SYNCING (x2)
• self._state.is_idle -> self._state == SyncState.IDLE (x1)

Rewrote _transition_to to inline the validity check now that
can_transition_to is gone. The original 5-edge transition table
becomes two invariants:

forbidden = new_state == self._state or (
    self._state == SyncState.IDLE and new_state == SyncState.SYNCED
)

Verified case-by-case against the original table — same semantics.

test_states.py — 135 -> 47 lines

Dropped tests for removed methods (can_transition_to, is_X).
Kept and tightened tests for the enum and accepts_gossip.

test_service.py — +10 lines

Added test_self_transition_raises_value_error to maintain coverage
of the no-self-transition invariant, replacing the parametrized test
that lived on can_transition_to. The IDLE -> SYNCED forbidden case
was already covered by an existing test.

Test plan

• ruff check, ruff format --check, ty check all pass
• pytest tests/lean_spec/subspecs/sync tests/lean_spec/subspecs/networking/test_network_service.py
  -> 166 passed
• All 9 (from, to) state pairs verified against the original
  transition table

Out of scope

The original review flagged other sync-layer items: _SyncStoreView
adapter, make_default_block_processor factory, and the
marketing-tone docstring on SyncService. Those stay queued for
follow-up PRs.

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