Hook up payload attestation message in fork choice spec tests

[terencechain]

The gloas on_payload_attestation_message fork choice spec tests were silently passing because the runner's Step/Check types had no fields for payload_attestation_message steps or the payload_timeliness_vote / payload_data_availability_vote checks, so YAML unmarshalling silently dropped both.

• Add payload_attestation_message step support: unmarshals the message and feeds it through ReceivePayloadAttestationMessage, asserting acceptance/rejection per the step'svalid field
• Add payload_timeliness_vote / payload_data_availability_vote check support, comparing the per-seat PTC vote bitvectors in forkchoice (null = no vote, true/false = recorded value)
• Add a PTCVotes getter on the doubly-linked-tree forkchoice to expose the vote bit vectors
• Convert the skip list to map[string]string so each skip logs its rationale

With the steps actually wired up, 2 of 7 test cases pass (from_block, not_ptc_member). The remaining 5 are skipped with rationales:

Test	Reason
valid, multiple_ptc_members_vote_independently	PTC votes are only recorded at the validator's first committee seat consensus-specs#5222 requires recording at every duplicate seat (balance-weighted committee)
current_slot_and_signature	Signature and current-slot checks live in gossip validation (validate_payload_attestation.go), not the fork choice handler
unknown_block_root	Unknown block root check lives in gossip validation (VerifyBlockRootSeen)
slot_mismatch	Block slot match check lives in gossip validation (VerifyBlockSlotMatches)