Bound accumulator amounts to representable limits before charging gas (#26956)

## Summary

- Reject transactions whose per-key accumulator totals are not
representable before charging gas: SUI balance keys are bounded by the
total SUI supply, other balances by `u64::MAX`
- Bound the cross-key total SUI withdrawn (gross Split) by the total
supply, since withdrawn SUI can be recombined outside the accumulator
(e.g. merged into the PTB gas coin)
- Accumulate SUI-conservation input/output totals in `u128` so
intermediate sums stay exact regardless of individual event amounts
- The guard turns a would-be arithmetic failure into a deterministic
`CoinBalanceOverflow` abort; it is ungated and applies uniformly across
protocol versions

## Test plan

- [ ] Added e2e regression tests covering per-key SUI/custom-coin
merges, single-withdrawal conservation, the gas-refund path, and
gas-coin recombination via `MergeCoins`
- [ ] `cargo simtest -p sui-e2e-tests address_balance`

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Co-authored-by: Mark Logan <mark@marklgn.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: 3 people