refactor(testing): consolidate consensus_testing genesis builders

[tcoratger]

Summary

Tidies up packages/testing/src/consensus_testing/genesis.py, which had grown a tangle of overlapping, inconsistently named builders. Collapses them into a small, consistent surface and removes duplication, updating all call sites.

The public surface goes from 7 symbols to 3 functions:

build_genesis_state(num_validators=4, *, genesis_time=Uint64(0), keyed=True, fork=LstarSpec())
build_anchor(num_validators, anchor_slot, *, fork=LstarSpec(), genesis_time=Uint64(0), keyed=True, synced=False)
build_genesis_store(num_validators=4, *, genesis_time=0, validator_index=ValidatorIndex(0), observer=False, keyed=True, time=None)

What changed

• Merged the keyed/zeroed builders. _build_validators (real XMSS keys) and make_validators (zeroed keys) were the same loop with a different key source; both now live inside build_genesis_state behind a keyed flag. generate_pre_state and make_genesis_state collapse into that one function too.
• Unified the verb. make_genesis_store → build_genesis_store; everything is now build_*.
• Removed reconstruct_block_from_header — a trivial helper; the genesis-block construction is inlined at its call sites.
• Single source for the genesis block. build_anchor now accepts anchor_slot=0 as the genesis case (its advance loop is simply empty there), so build_genesis_store delegates to it. The genesis-block construction now exists in exactly one place, and the if anchor_slot == 0 dodge branches in the sync/api_endpoint fixtures collapse away.
• Inlined the former module-level defaults (_DEFAULT_GENESIS_TIME, _DEFAULT_VALIDATOR_INDEX) and defaulted fork directly in the signatures. The immutable Uint64/ValidatorIndex/LstarSpec calls are whitelisted in ruff's flake8-bugbear.extend-immutable-calls so the in-signature defaults pass B008.
• Updated all call sites across tests/ and packages/.

No backward-compatibility shims — old names are removed, not aliased.

Testing

• just check passes (ruff lint + format, ty, codespell, mdformat, lock).
• Unit tests across the affected node modules pass (test_anchor, test_node, chain/test_service, api/endpoints, validator/test_service).

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