PR Body

## Hosting API: Bounded-Wait Timeouts

### Summary

Replaces every unbounded `WaitOne()`/`Wait()` in the PowerShell hosting API with bounded alternatives.
Adds two opt-in public API members for callers that need predictable failure modes.
All existing code with default settings is byte-for-byte identical to previous behaviour.

### Changes (6 files, +173 / −13)

| File | Change |
|------|--------|
| `PowerShell.cs` | `PSInvocationSettings.Timeout`, `Stop(TimeSpan)`, Phase 2 bounded Invoke, pool + batch timeouts |
| `ConnectionBase.cs` | Parallel `StopPipelines(TimeSpan)`, 30s runspace-open wait |
| `LocalConnection.cs` | 30s close/job waits, `Dispose()` catches `TimeoutException` → Broken state |
| `LocalPipeline.cs` | 30s `PipelineFinishedEvent.WaitOne` |
| `PowerShellStrings.resx` | `OperationTimedOut`, `StopTimedOut` messages |
| `RunspaceStrings.resx` | `StopPipelinesTimedOut` message |

### New Public API (RFC required)

| Member | Signature | Default |
|--------|-----------|---------|
| `PSInvocationSettings.Timeout` | `TimeSpan { get; set; }` | `Timeout.InfiniteTimeSpan` |
| `PowerShell.Stop(TimeSpan)` | `void Stop(TimeSpan timeout)` | — |

### Test Coverage

- 19 xUnit C# tests (`test/xUnit/csharp/test_Timeout.cs`)
- 15 Pester PowerShell tests (`test/powershell/engine/Api/Timeout.Tests.ps1`)
- 8 adversarial scenario scripts — all PASS (Run 2, 2026-03-12)

### Breaking Changes

None. `PSInvocationSettings.Timeout` defaults to `InfiniteTimeSpan`. Any code that does not set
`Timeout` takes the original same-thread code path with zero overhead.

**STA COM caveat:** When `Timeout` is finite, `Invoke()` uses `Task.Run` (MTA thread). Scripts
that depend on STA COM apartment state must leave `Timeout` at its default.

### Checklist

- [x] Tests added (`test_Timeout.cs`, `Timeout.Tests.ps1`)
- [x] Documentation updated (companion site linked above)
- [x] Changelog updated (`CHANGELOG.md` entry for vNext)
- [x] No breaking changes
- [ ] RFC filed (required for `PSInvocationSettings.Timeout` + `Stop(TimeSpan)`)