power.routeros: power_on must promote saved 'off' to 'forced-on'

[widgetii]

Why

`RouterOSController` saves the port's previous `poe-out` mode on `power_off` so `power_on` can put it back where it was — that preserves "auto-on" vs "forced-on" distinctions correctly. But if the port was already "off" when `power_off` ran (recovering a parked camera; bench is dark on a fresh shell), the saved "previous" mode is literally "off", and `power_on` then "restored" the port to off. Cycling a powered-down port left it powered down forever, which silently broke every recovery flow that started from off.

Surfaced while testing #103 — every fresh script that started with `power_off → power_on` left the camera unpowered, until I switched to `_set_poe(port, 'forced-on')` directly as a workaround.

What

`power_on` must always result in a powered port. If the saved mode is "off", promote to "forced-on" and log the promotion (visible at `-v`):

```python
async def power_on(self, port: str) -> None:
restore_mode = self._saved_poe_out.pop(port, "forced-on")
if restore_mode == "off":
logger.info("PoE ON: %s on %s (saved state was 'off' — promoting to 'forced-on')", port, self._host)
restore_mode = "forced-on"
else:
logger.info("PoE ON: %s on %s (restoring %s)", port, self._host, restore_mode)
await self._set_poe(port, restore_mode)
```

Test plan

• `uv run pytest tests/ -x --ignore=tests/fuzz` — 527 passed, 2 skipped (5 new tests)
• `uv run ruff check` + `mypy` on changed files — clean
• Verified on real hardware (MikroTik 10.216.128.2 / ether8, currently powered down):
  ```
  before: ether8 poe-out='off'
  power_off: saved='off'
  power_on: saved=None, port now 'forced-on'
  ```

5 new tests in `tests/test_power.py::TestRouterOSPowerOnOff` covering forced-on round-trip, auto-on round-trip (preserved — not blindly clobbered to forced-on), off→on promotion (the bug), no-prior-off default, and double-power_off-doesn't-clobber-saved-state edge cases. They use a `_PoeStateRouterOS` subclass that stubs the two network primitives so we can exercise the save/restore state machine without a real switch.

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