Remove ExecV3: Refactor BlockAssembler to accept StateWriter

## Sub-task of #19318

### Phase 2: Refactor BlockAssembler

**File**: `execution/exec/block_assembler.go`

The `BlockAssembler` currently hardcodes `NoopWriter` at 3 locations:

| Method | Line | Writes via |
|--------|------|-----------|
| `Initialize()` | 135 | `InitializeBlockExecution` → `FinalizeTx(rules, writer)` |
| `AddTransactions()` | 165,216 | `ApplyTransaction` → `FinalizeTx(rules, writer)` |
| `AssembleBlock()` | 328 | `FinalizeBlockExecution` → `CommitBlock(rules, writer)` |

### Changes

1. Add `stateWriter state.StateWriter` field to `BlockAssembler`
2. Modify `NewBlockAssembler` to accept optional `StateWriter` parameter (nil → `NoopWriter` for backward compatibility)
3. Add `OnTxCommitted func()` callback field — called after each successful user tx commit (used by caller to advance txNum)
4. Replace 3 hardcoded `NoopWriter` usages with `ba.stateWriter`
5. In `AddTransactions`, after each successful `commitTx`, call `ba.OnTxCommitted()` if set

When `NoopWriter` is used (default), behavior is identical to today. When a real `Writer` is passed, `FinalizeTx` and `CommitBlock` write state changes to `SharedDomains`.

### Key state write flow

- `ApplyTransaction` → `applyTransaction` → `ibs.FinalizeTx(rules, stateWriter)` (state_processor.go:99)
- `FinalizeBlockExecution` → `ibs.CommitBlock(rules, stateWriter)` (block_exec.go:350)
- `InitializeBlockExecution` → `ibs.FinalizeTx(rules, stateWriter)` (block_exec.go:371)

`FinalizeTx` writes per-tx dirty state objects. `CommitBlock` writes remaining dirty state at block end. They're complementary — `FinalizeTx` clears the journal after writing.