Stage H: py3.9 / numpy 1.22 / scipy 1.8 / control 0.9 compatibility + a py3.9 CI leg

**Stage H (spec §8).** Deliberately **not on the critical path** — see the measurement below.

The hardware path is pinned to the site CDS baseline (**python 3.9.13 / numpy 1.22.4 / scipy 1.8.1 /
control 0.9.2 / slycot 0.4.0.0**) because the working environment is a clone of the site CDS environment
and the compiled extensions are built against that ABI.

### The decisive measurement

**The CDS-relevant half of this repo already passes on that baseline.** `test_step5_safety`,
`test_step7_loop`, `test_step8_cli`, `test_periodic_measurement`, `test_rtsfreerun_backend`,
`test_excitation`, `test_step4_twin`, `test_step6_estimator`, `test_step12_ml_estimator`,
`test_resolution` → **61 passed / 1 skipped**.

The full suite does **not** (250 passed / 35 failed / 12 skipped / 4 errors), but every failure is in the
DARM / MIMO / SOS / arcade / playground half, which this campaign does not touch. So the deployment gate is
that **named subset**, not "the suite passes".

### Four root causes — and two need no shim

| break | resolution | sites |
|---|---|---|
| `FrequencyResponseData.frdata` absent before control 0.10.2 (it was `.fresp`) | **the one genuine shim** — a `_frd(sys, w)` helper trying `frdata` then `fresp` (prefer `frdata`; `.fresp` emits `FutureWarning` on 0.10) | [`src/system_ident/darm_actuation.py:144`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/darm_actuation.py#L144), [`src/system_ident/darm_actuation.py:208`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/darm_actuation.py#L208), [`src/system_ident/closed_loop_id.py:37`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/closed_loop_id.py#L37) + 4 test sites → 7 calls collapse to 1 |
| **`control.tf2ss(StateSpace)` rejected on 0.9.2**, accepted on 0.10 | `control.ss(x)` — works on **both** | [`src/system_ident/mimo_loop.py:31`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/mimo_loop.py#L31), [`src/system_ident/mimo_plant.py:40`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/mimo_plant.py#L40), [`src/system_ident/mimo_plant.py:72`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/mimo_plant.py#L72), [`src/system_ident/sos_closed.py:70`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/sos_closed.py#L70), [`src/system_ident/darm_actuation.py:124`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/darm_actuation.py#L124), [`tests/test_mimo.py:236`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/tests/test_mimo.py#L236) — 11 failures |
| `np.trapezoid` is numpy ≥ 2 only | `from scipy.integrate import trapezoid` (scipy ≥ 1.6) — already this repo's `src/` convention ([`src/system_ident/design/pintelon.py:18`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/design/pintelon.py#L18)) | **zero `src/` sites**; [`tests/test_arcade.py:50`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/tests/test_arcade.py#L50), `tests/test_playground.py`, `experiments/realistic/*.py`, `docs/*_reference.py` |
| `plotly` absent | environment, not version | 3 collection errors |

Plus: fix the **order-dependent circular import** at [`src/system_ident/backends/darm_adapter.py:14`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/backends/darm_adapter.py#L14)
← [`src/system_ident/darm.py:506`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/darm.py#L506) (bottom-imports `DARMBackend`). Move it into the function that needs
it or a `TYPE_CHECKING` block. Verified order-dependent, **not** version-dependent — it reproduces
identically on py3.12, and it will bite whoever first writes `import system_ident.backends.darm_adapter`
at the top of a script.

### Do not

- **Raise the floor** — impossible, CDS 3.1.2 is py3.9-only.
- **Build a general `_compat.py`** — one three-line helper plus two mechanical renames. Version-conditional
code at 7 sites would drift; a single helper cannot.

### Also

After the `control.ss` rename, run [`tests/test_mimo.py`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/tests/test_mimo.py) on control 0.10.2 too, to confirm the
`LinearIOSystem` vs `StateSpace` return type does not matter downstream.

### CI

Add a **py3.9 leg**. CI runs 3.12 only ([`.github/workflows/ci.yml:26`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/.github/workflows/ci.yml#L26)) and executes exactly one test
file, so a 3.9 regression is invisible until it fails on deployment.

---
**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.*

Start with the named CDS-relevant test subset and the compatibility sites in src/system_ident/darm_actuation.py, closed_loop_id.py, mimo_loop.py, mimo_plant.py, sos_closed.py, and backends/darm_adapter.py. Check .github/workflows/ci.yml and tests/test_mimo.py, then verify the subset on Python 3.9 and control 0.10.2; done means the compatibility fixes, import-order issue, and Python 3.9 CI leg are covered.

Written by the indexing model from the issue text.