perf(serde_snapshot): build versioned epoch stakes in thread

[kskalski]

Problem

#9831 changed deserialization of epoch stakes to intermediate representation as cheaper vectors, but at some point the final im::HashMap need to be constructed, which is a slow CPU-bound process (can take even >8s). Current this is done serially after accounts db index is built.

Since this part of bank data initialization is independent from calculating accounts db, it could be done in separate thread such that it's ready once accounts db reconstruction is done.

Summary of Changes

• change versioned_epoch_stakes in BankFieldsToDeserialize to Vec (this makes deserialization even slightly cheaper), since it's intermediate representation that is only iterated on later
• create final HashMap<Epoch, VersionedEpochStakes> in separate thread spawned before reconstruct_accountsdb_from_fields
• pass the map to Bank::new_from_snapshot to be used instead of reading from fields.versioned_epoch_stakes

Performance change

Time between loading stakes and finishing new_from_snapshot (rent_collector log line) shrinks from 4.22s to 0.011s

PR

[2026-01-14T06:53:03.140175595Z INFO  solana_runtime::bank] Loading Stakes took:  took 2.4s
[2026-01-14T06:53:03.151382107Z INFO  solana_runtime::serde_snapshot] rent_collector: RentCollector { epoch: 910, epoch_schedule: EpochSchedule { slots_per_epoch: 432000, leader_schedule_slot_offset: 432000, warmup: false, first_normal_epoch: 0, first_normal_slot: 0 }, slots_per_year: 78892314.984, rent: Rent { lamports_per_byte_year: 3480, exemption_threshold: 2.0, burn_percent: 50 } }
[2026-01-14T06:53:03.151403837Z INFO  solana_runtime::snapshot_bank_utils] rebuild bank from snapshots took 61.1s

master

[2026-01-14T07:11:55.046225292Z INFO  solana_runtime::bank] Loading Stakes took:  took 2.5s
[2026-01-14T07:11:59.261691409Z INFO  solana_runtime::serde_snapshot] rent_collector: RentCollector { epoch: 910, epoch_schedule: EpochSchedule { slots_per_epoch: 432000, leader_schedule_slot_offset: 432000, warmup: false, first_normal_epoch: 0, first_normal_slot: 0 }, slots_per_year: 78892314.984, rent: Rent { lamports_per_byte_year: 3480, exemption_threshold: 2.0, burn_percent: 50 } }
[2026-01-14T07:11:59.261725947Z INFO  solana_runtime::snapshot_bank_utils] rebuild bank from snapshots took 66.3s

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.5%. Comparing base (729712b) to head (5e174c5).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #10002   +/-   ##
=======================================
  Coverage    82.5%    82.5%           
=======================================
  Files         844      844           
  Lines      316758   316780   +22     
=======================================
+ Hits       261579   261622   +43     
+ Misses      55179    55158   -21     

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[brooksprumo]

Looks good!

[brooksprumo]