refactor: order Checkpoint by slot to drop max() lambda noise

[tcoratger]

Summary

Adds Checkpoint.__lt__ ordered by slot so the four max(...) calls in forkchoice/store.py can drop their key=lambda c: c.slot argument.

Tie-break semantics preserved

Equal-slot checkpoints are intentionally incomparable (a < b and b < a both return False). Python's max() keeps its first argument when neither is strictly less, which matches what each call site already relied on:

• on_block: max(self.X, post_state.X) — store wins ties (the documented "anchor-pinned, authoritative on ties" invariant).
• produce_block_with_signatures: max(final_post_state.X, store.X) — post-state wins ties (existing behavior; not changed in this PR).

No behavioral change at any call site. Just the lambda noise removed.

Tests

tests/lean_spec/subspecs/containers/test_checkpoint.py — 8 tests, no duplicate scenarios:

• Strict-less branch returns True for lower slot
• Strict-less branch returns False for higher slot
• Equal slots with different roots are incomparable both directions (verifies root is ignored)
• Anti-reflexivity: a checkpoint is not less than itself
• Direct dunder call returns the NotImplemented sentinel for foreign types
• < operator raises TypeError after Python's reflected fallback fails
• max() picks the higher-slot checkpoint regardless of argument order
• max() keeps the first argument on slot ties (the property the forkchoice store relies on)

Test plan

• uvx tox -e all-checks passes (ruff, format, ty, codespell, mdformat).
• uv run pytest tests/lean_spec/subspecs/containers/test_checkpoint.py — 8 passed.
• Reviewer to spot-check tie-break preservation at each call site.

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