refactor(chain): tidy ChainService and rework its test suite

[tcoratger]

Summary

Cleans up the consensus clock driver (ChainService) and replaces its mock-heavy test suite with a clearer, real-object suite that covers the full scenario spread.

Service (src/lean_spec/node/chain/service.py)

• Reuse SlotClock's own pre-genesis wait (sleep_until_next_interval) instead of recomputing the wait by hand — one home for that timing, and it no longer truncates the fractional remainder.
• Remove the redundant no-op store reassignment in the skip-ahead path (the real Store mutates in place), and drop the needless int() cast around INTERVALS_PER_SLOT.
• Add a forward-only precondition (assert target_interval >= store.time) documenting the invariant the catch-up loop relies on, plus a note that attestation acceptance for jumped-over slots defers safely to the final slot's tick.
• Rewrite docstrings/comments to the repo documentation rules (one sentence per line, no identifier names in prose, WHY over what).

Tests (tests/lean_spec/node/chain/test_service.py)

• Drop the MockStore / MockCheckpoint / StoreInterceptingSpec scaffolding in favor of a real Store plus a thin recording spec (ProbeSpec) and a 3-field SyncServiceStub naming exactly what the driver touches.
• Remove three tests that exercised SlotClock.sleep_until_next_interval rather than ChainService — already covered in test_clock.py (test-mirrors-source rule).
• Call the catch-up helpers (_tick_to, _initial_tick) directly where they return; only drive the infinite run() loop in the handful of tests where the loop itself is the subject.
• Cover the scenario spread with parametrized cases: no-op (target == now), within-slot, exact slot boundary, just-over, deep skip (4 and 20 slots); plus edge cases — the backward-target precondition, and a store swapped in during the yield (verifies the post-yield re-read).

Verification

• 22 tests, 100% line and branch coverage of service.py.
• The two new guard tests are mutation-checked: removing the post-yield re-read or the precondition assert makes the targeted test fail.
• just check passes (lint, format, type check, spell, mdformat).

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