Stage E: an out-of-band STOP path must be mandatory on the hardware backend

**Stage E (spec §5).**

[`src/system_ident/cli.py:118`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/cli.py#L118) is the **only** place `watchdog.abort("operator STOP")` is wired from
outside the loop — and it lives in the dashboard. So "the dashboard is optional" is true for the twin and
**false for hardware**: running headless on hardware means there is no STOP button, and per the lifecycle
issue Ctrl-C does not currently stop the excitation either. That combination is not acceptable on a real
suspension.

### Do NOT force `--no-dashboard`

[`src/system_ident/cli.py:108`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/cli.py#L108) already degrades gracefully when the extra is missing (`find_spec`
check → prints a note → runs headless), which is the right behaviour. And the extra is installable on the
deployment machine: `fastapi` / `uvicorn` / `websockets` are pure-Python, work under py3.9, and are already
declared at [`pyproject.toml:31`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/pyproject.toml#L31). `pydantic` being absent from the deployment lock is irrelevant — it
is never imported directly.

### Fix

Require *a* stop path when the CDS backend is selected — either:

- the dashboard is running, **or**
- the SIGINT handler from the lifecycle issue is installed **and** its use is printed prominently at
startup (e.g. "no dashboard: press Ctrl-C once to ramp down and hand off").

Refuse to start a hardware run with neither.

---
**Campaign:** CDS hardware backend · branch [`feat/cds-hardware-backend`](https://github.com/CaltechExperimentalGravity/system_ident/tree/feat/cds-hardware-backend)
· [spec](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/docs/superpowers/specs/2026-08-03-cds-hardware-backend-design.md) · [plan](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/docs/superpowers/plans/2026-08-03-cds-hardware-backend.md) · [handoff](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/notes/cds-hardware-bringup-2026-08.md)
*Code is deferred until the plan and issues have been reviewed.*

Read src/system_ident/cli.py around lines 108-118 and trace the lifecycle SIGINT handler referenced by the issue. Run the CDS hardware CLI in dashboard and headless modes; done means headless startup clearly advertises Ctrl-C and a hardware run refuses to start when neither the dashboard nor the SIGINT stop path is available.

Written by the indexing model from the issue text.