IBS 2-Cache Phase 2b: Remove BalanceIncreaseSet from serial/parallel applyUpdates path

Part of #19623 — Rationalize IntraBlockState to a 2-Cache Model.
Depends on #19701 (Phase 2).
Required before #19703 (Phase 4) which removes the `balanceInc` map from IBS.

## Problem

`IntraBlockState.balanceInc` tracks balance increases that arrive without a prior account read (e.g. miner reward, block reward transfers). After a tx, `ibs.BalanceIncreaseSet()` exports these as a separate `map[Address]uint256.Int` that flows through:

```
ibs.BalanceIncreaseSet()
→ txtask.BalanceIncreaseSet (exec/txtask.go:571)
→ rs.applyUpdates(... balanceIncreases ...) (state/rw_v3.go:62)
→ SharedDomains
```

The parallel path (Phase 2, #19701) derives balance changes directly from `VersionedWrites`, making this separate map redundant. But it is still consumed by the **serial path** (`exec3_serial.go:521`) and by `rs.applyUpdates`. Phase 4 (#19703) removes `balanceInc` from IBS; this phase must happen first.

## Changes

- In `StateUpdatesFromVersionedWrites` (introduced in Phase 2), ensure `BalancePath` writes for all addresses — including miner/reward addresses — are captured. These currently arrive via `AddBalance` which goes through `balanceInc` when the account doesn't exist yet.
- Remove the `balanceIncreases map[accounts.Address]uint256.Int` parameter from `rs.applyUpdates()` in `rw_v3.go`. Merge its logic into the main account update loop (balance increases for accounts not otherwise modified become a synthesized `BalancePath` `VersionedWrite`).
- Update `exec3_serial.go` to stop passing `txTask.BalanceIncreaseSet` into the apply path.
- Remove `BalanceIncreaseSet` field from `TxTask` in `exec/txtask.go`.
- Remove `ibs.BalanceIncreaseSet()` method from `IntraBlockState` (or leave as dead stub until Phase 4 removes `balanceInc` entirely).

## New feature tests

- **`TestBalanceIncreaseInVersionedWrites`** — a tx that only increases a miner balance (no prior read) produces a `BalancePath` entry in `VersionedWrites` and is correctly reflected in `StateUpdates` from `StateUpdatesFromVersionedWrites`.
- **`TestApplyUpdatesNoBalanceIncreaseParam`** — drive `rs.applyUpdates` without a separate `balanceIncreases` arg; assert domains have correct final balances for reward-only accounts.

## Regression tests

- **`TestApplyVersionedWrites`** (existing) — must pass; miner/reward balance changes must be present.
- **`TestStateUpdatesEquivalence`** (Phase 2, un-skipped) — must still pass after this change.
- Run `go test ./execution/stagedsync/...` — serial executor produces identical state roots before and after.
- Run `go test ./execution/state/...` — no failures.
- `make test-short` passes.

## Files affected

- `execution/state/rw_v3.go` — remove `balanceIncreases` param from `applyUpdates`
- `execution/exec/txtask.go` — remove `BalanceIncreaseSet` field
- `execution/stagedsync/exec3_serial.go` — stop passing `BalanceIncreaseSet`
- `execution/stagedsync/exec3_parallel.go` — any remaining `BalanceIncreaseSet` references

## Acceptance criteria

- `BalanceIncreaseSet` field removed from `TxTask`
- `applyUpdates` takes no separate `balanceIncreases` parameter
- Serial and parallel paths produce identical state roots to before
- `make test-short` passes

Start with execution/state/rw_v3.go, execution/exec/txtask.go, and execution/stagedsync/exec3_serial.go; trace applyUpdates and the BalanceIncreaseSet references through the serial and parallel paths. Run TestBalanceIncreaseInVersionedWrites, TestApplyUpdatesNoBalanceIncreaseParam, TestApplyVersionedWrites, and TestStateUpdatesEquivalence, then run the listed package tests and make test-short. Done means the separate field and parameter are removed and state roots remain equivalent.

Written by the indexing model from the issue text.