perf(vm): avoid duplicate LockedCoins read in GetAccount

[0u-Y]

Description

GetAccount computed spendable and locked via separate SpendableCoin and
lockedCoin calls, each reading LockedCoins from the bank store — so every
GetAccount read LockedCoins twice. This reads balance and LockedCoins
once in a new spendableAndLockedCoin helper and derives both; the now-unused
lockedCoin is removed.

Equivalence

• locked: LockedCoins(addr).AmountOf(evmDenom) — unchanged.
• spendable: balance.SubAmount(locked) on the sdk.Coin from
  bankWrapper.GetBalance, identical to what bankWrapper.SpendableCoin
  computes (same sdk.Coin.SubAmount, same panic-on-negative).

validateCoinInfo enforces Denom == ExtendedDenom + 18 decimals, so
AmountOf(evmDenom) == AmountOf(extendedDenom); the values and the #1187
locked-snapshot semantics are preserved.

Benchmarks

Transfer-saturated block, local harness (not in-tree, happy to share),
2000 txs/block, -benchtime=10x -count=6, Ryzen 9 7900 (n=6):

benchmark	sec/op	B/op	allocs/op
Sequential	677.5m → 661.8m (−2.32%, p=0.004)	393.3Mi → 380.5Mi (−3.26%)	6.030M → 5.758M (−4.51%)
BlockSTM	206.5m → 209.6m (~, p=0.82)	464.5Mi → 452.3Mi (−2.63%)	6.954M → 6.692M (−3.77%)

p=0.002 unless noted. Allocations drop significantly on both paths; serial
wall-time −2.3%; Block-STM wall-time is within noise. Workload-dependent —
smaller for contract-heavy blocks.

Tests

TestKeeperTestSuite (StateDB balance suite) and the staking-precompile suite
(#1187 locked-via-precompile guard) pass unchanged.

The merged helper no longer opens the SpendableCoin/LockedCoin tracing
sub-spans (the parent GetAccount span is unchanged).

Closes: #1202

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Author Checklist

I have...

• tackled an existing issue or discussed with a team member — GetAccount performs a redundant LockedCoins read #1202
• left instructions on how to review the changes — see Equivalence above
• targeted the main branch

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