feat: SIMD-0185 vote state v4 support

[7layermagik]

Note: This PR depends on #208 (SIMD-0321) as its base. Merge that first.

Summary

Implements SIMD-0185 (Vote State V4), the next structural revision of the on-chain vote account format. V4 removes PriorVoters, changes commission from u8 percentage to u16 basis points, and adds new fields for revenue splitting and BLS keys. Gated behind feature vote_state_v4 (Gx4XFcrVMt4HUvPzTpTSVkdDVgcDSjKhDN1RqRS6KDuZ).

Changes

New struct and serialization (pkg/sealevel/vote_state.go)

• VoteState4 struct with all V4 fields: InflationRewardsCollector, BlockRevenueCollector, InflationRewardsCommissionBps (u16), BlockRevenueCommissionBps (u16), PendingDelegatorRewards, BlsPubkeyCompressed (Option<[u8;48]>), plus the standard fields (NodePubkey, AuthorizedWithdrawer, Votes as LandedVote deque, RootSlot, AuthorizedVoters, EpochCredits, LastTimestamp).
• Discriminant 3 (VoteStateVersionV4) added to the existing iota enum (V0_23_5=0, V1_14_11=1, Current/V3=2, V4=3).
• Full UnmarshalWithDecoder and MarshalWithEncoder matching Agave's frame_v4 layout byte-for-byte.

VoteStateVersions switch sites

All 6 switch sites in VoteStateVersions updated with V4 cases:

• UnmarshalWithDecoder / MarshalWithEncoder — delegates to VoteState4 codec
• IsInitialized — checks V4 AuthorizedVoters length
• ConvertToCurrent — converts V4 to the VoteState working struct, preserving V4-specific fields via internal preservation fields (wasV4, v4InflationRewardsCollector, etc.)
• LastTimestamp / NodePubkey — direct field access on V4

V4 field preservation through the processing loop

Mithril processes all vote state versions through a common VoteState (V3) working struct. V4-only fields (collectors, bps commission, pending rewards, BLS key) are preserved via unexported fields on VoteState. On write-back:

• wasV4 = true: preserved original V4 values are restored (no lossy round-trip)
• wasV4 = false (first V3→V4 conversion): Agave-matching defaults are used:
  • InflationRewardsCollector = vote account pubkey
  • BlockRevenueCollector = node pubkey
  • InflationRewardsCommissionBps = u8_commission * 100
  • BlockRevenueCommissionBps = 10000 (100%)
  • PendingDelegatorRewards = 0, BlsPubkeyCompressed = None

Write path (setVoteAccountState)

When VoteStateV4 feature is active (checked BEFORE VoteStateAddVoteLatency):

• Serializes as discriminant 3 using newVoteState4FromCurrent
• Same allocation size as V3 (3762 bytes) — no resize needed for V3→V4
• No V1_14_11 fallback on resize failure — returns InstrErrAccountNotRentExempt per SIMD-0185
• Commission updates via VoteProgramUpdateCommission sync the preserved v4InflationRewardsCommBps field

Behavioral changes

• Authorized voter purge policy: V4 purges at currentEpoch - 1 (retains one extra epoch), vs V3's currentEpoch. Implemented in GetAndUpdateAuthorizedVoter which now accepts features.Features.
• PriorVoters tracking: SetNewAuthorizedVoter skips PriorVoters.Append when V4 is active (V4 has no PriorVoters field). Both methods now accept features.Features.

External consumer updates

• pkg/sealevel/vote_program.go: verifyAndGetVoteState signature updated to accept features; all 3 callers (VoteProgramProcessVote, VoteProgramProcessVoteStateUpdate, VoteProgramProcessTowerSync) pass features through. VoteProgramAuthorize passes features to SetNewAuthorizedVoter.
• pkg/rewards/rewards.go: voteCommissionSplit reads V4 InflationRewardsCommissionBps / 100; calculateStakePointsAndCredits reads V4 EpochCredits.
• pkg/replay/transaction.go: recordVoteTimestampAndSlot reads V4 LastTimestamp.
• pkg/snapshot/manifest_decoder.go: Snapshot vote account loading handles V4 LastTimestamp and NodePubkey.

Correctness notes

• Commission round-trip: u8 × 100 → u16 bps; bps / 100 → u8. No precision loss for values 0–100. Original bps preserved exactly for V4→V4 round-trips.
• V4 immutable fields: InflationRewardsCollector, BlockRevenueCollector, PendingDelegatorRewards, BlsPubkeyCompressed — no vote program instruction modifies these in SIMD-0185, so compute-on-first-write defaults are correct.
• Agave references: agave/programs/vote/src/vote_state/handler.rs (convert_to_v4, V4 purge, V4 set_new_authorized_voter skips prior_voters)

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