refactor(forks): tighten ForkProtocol surface (Stage 1 of #686)

[tcoratger]

Part of #686 — Stage 1 only. Other stages (2–8) remain in scope of the tracking issue.

Summary

Tightens the ForkProtocol surface so subclass-and-narrow inheritance ("Vision B") works under pyright/ty invariance rules, and removes the silent fork bypass in pre-state generation.

Type tightening

• Drop Any from ForkProtocol method signatures. Replace with Uint64 and structural Protocols.
• Add SpecBlockType Protocol so block_class is no longer typed type (i.e. type[Any]).
• Drop ClassVar from state_class / block_class / store_class. ClassVar triggers the type checker's invariance rule and would block a future Devnet5Spec(LstarSpec) from declaring state_class: type[Devnet5State] = Devnet5State. ClassVar stays on true constants (NAME, VERSION, GOSSIP_DIGEST, previous).
• Rename NETWORK_NAME → GOSSIP_DIGEST. The field already served the gossipsub fork-digest role; the rename matches the intent.

generate_pre_state fork fallback

• Remove the fork: ForkProtocol | None = None branch that called State.generate_genesis(...) directly when fork was None. Every call now dispatches through fork.generate_genesis(...).
• The pre pytest fixture (packages/testing/.../filler.py) now requests the framework's fork fixture and threads fork.spec_class()() through.
• Direct callers in fixture builders (sync.py, api_endpoint.py) pass LstarSpec() explicitly, consistent with their existing lstar coupling.

Honest casts at concrete-fork boundaries

fork.generate_genesis(...) and fork.create_store(...) return Protocol-typed values (SpecStateType / SpecStoreType). The two callers that need narrow access (__main__.py, node.py) use cast(State, ...) / cast(Store, ...) at the boundary — no # type: ignore needed. The lstar fork class deliberately does not override these methods to "narrow" the return type, since doing so would require # type: ignore[arg-type] to bridge the LSP-widened input back to the concrete narrow signature. A clean cast at the boundary is more honest than an ignore in the spec class.

Stage 1 checklist (from #686)

• Replace Any in ForkProtocol method signatures with fork-stable primitives and structural protocols.
• Add SpecBlockType Protocol so block_class stops being typed type.
• Drop ClassVar from state_class / block_class / store_class.
• Add GOSSIP_DIGEST: ClassVar[str]; route consumers through fork.GOSSIP_DIGEST.
• Delete the generate_pre_state fork fallback. Thread fork through the pre pytest fixture.

The remaining Stage 1 items (use DEFAULT_REGISTRY in __main__.py, add previous, rename SpecRunner → ForkRegistry, make upgrade_state abstract, direct bindings on devnet5/spec.py) were already merged in earlier PRs.

Test plan

• uvx tox -e all-checks passes (ruff, format, ty, codespell, mdformat).
• uv run pytest --no-cov passes.
• uv run fill --fork=lstar --clean -n auto generates fixtures.

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