refactor: simplify testing framework with proper OOP design

[tcoratger]

Summary

• Move building logic from fixture classes to spec types where it belongs. Each spec type now owns the methods that operate on its own fields.
• Fixture files are now pure orchestration (~357 lines for fork_choice, ~144 for verify_signatures)
• Fix bugs (double fixture registration, assert tuple, truncated hex in errors)
• Remove dead code, modernize typing, improve documentation

What changed

Spec types own their building logic:

Spec Type	New Methods
BlockSpec	resolve_proposer_index, resolve_parent_root, build_attestations, build_signed_block, build_signed_block_with_store
AggregatedAttestationSpec	build_attestation_data, build_invalid_proof
GossipAttestationSpec	build_attestation_data, build_signed
GossipAggregatedAttestationSpec	build_attestation_data, build_signed

Framework cleanup:

• Fix double fixture registration bug in BaseFixture
• Move expect_exception field + serializer to BaseConsensusFixture
• Rewrite StoreChecks.validate_against_store with explicit checks (no dispatch tables, no unused args)
• Convert all non-self methods to @staticmethod
• Modernize typing imports (dict, list, type, set)
• Delete dead code (_children, supports_fork, transitions_to)
• Add XmssKeyManager.sign_and_aggregate for common pattern
• Create test_types/utils.py for shared resolve_checkpoint
• Update doc-writer agent rules
• Show full hex in error messages

Net effect

• -465 lines net reduction (1368 added, 1833 removed)
• fork_choice.py: 666 → 357 lines
• verify_signatures.py: 370 → 144 lines
• Zero duplication between fixture files

Test plan

• uvx tox -e all-checks passes
• uv run fill --fork=devnet --clean -n auto generates all 123 fixtures

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