# K09 — scheduler facade and scoped kernel guards

K09 separates a process-wide service from genuinely scoped operations:

- `Scheduler` is a non-constructible static facade for start, state,
  suspend and resume;
- `this_task` exposes the current handle, tick count and context yield;
- `CriticalSection` pairs `taskENTER_CRITICAL` with
  `taskEXIT_CRITICAL`;
- `SchedulerSuspendGuard` pairs `vTaskSuspendAll` with
  `xTaskResumeAll`.

Both guards are non-copyable and non-movable. Their destructors restore state
on three distinct early-return paths. Always-inline methods leave no wrapper
symbols or virtual/runtime machinery in the target ELF.

## Measured Hazard3 result

```text
critical nesting: 0 -> 1 -> 2 -> 1 -> 0
MIE bit:           1 -> 0 -> 0 -> 0 -> 1
suspend scheduler: state running -> suspended -> running
MIE while suspended: 1 (interrupts remain enabled)
nested guards:     running -> suspended -> suspended -> suspended -> running
```

All three critical paths return with nesting 0 and MIE 1. All three scheduler
paths return in `running` state with MIE 1. The current handle seen through
`this_task` equals the native worker handle.

## Build and evidence

```sh
make check
```

Expected:

```text
PASS host: static facade, this_task and balanced guards on all exits
PASS ABI: facade/guards inline; no vtable/runtime; mstatus transitions present
PASS task01: kernel facade + scoped guards restore all paths
```

## Debug

```gdb
b kernel_facade_debug_checkpoint
p g_raw_critical_depth
p g_raw_critical_mie
p g_guard_critical_after_depth
p g_guard_critical_after_mie
p g_raw_scheduler_state
p g_guard_scheduler_after_state
p g_nested_scheduler_state
p g_kernel_facade_pass
```

## Scope boundaries

Scheduler suspension prevents context switches but does not disable interrupts.
Do not call a potentially blocking API while the scheduler is suspended.
`CriticalSection` is a task-context guard, not an ISR mutex. `Scheduler` itself
is not RAII because scheduler start has no useful lexical lifetime and normally
does not return.