[deferred] Foton ZPK/SOS export + provenance manifest for a delivered fit

**Component 2 — deferred.**

A fit is only useful to the control room if it can be **dropped into Foton** and if its provenance is
auditable.

### Export

Foton ZPK / SOS export of the delivered model. Two traps recorded from prior work:

- Foton's "f"-plane sign conventions differ from "s"; scientific notation needs an explicit `+`.
- Do not expand an SOS cascade into a high-order polynomial. The sibling project found a 32nd-order
filter's coefficients span ~40 decades, which makes the information matrix singular — **refitting beats
truncating** (measured 3e-4 vs 38% error). Work from the SOS.

### Provenance manifest

Every real hardware number must arrive through
[`src/system_ident/provenance.py`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/src/system_ident/provenance.py) `record(name, value, kind, source, unit=)` with
`kind ∈ {MEASURED, PAPER, USER, CONSTANT, DERIVED}`. **`ASSUMED` fails CI** — and note that
[`tests/test_provenance.py`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/tests/test_provenance.py) is the *only* test CI runs ([`.github/workflows/ci.yml:26`](https://github.com/CaltechExperimentalGravity/system_ident/blob/feat/cds-hardware-backend/.github/workflows/ci.yml#L26)), so this
is the one gate that is always enforced.

That makes the manifest cheap to add and expensive to skip: counts↔N calibration, DAC limits, front-end
rate and channel names all become recorded, sourced values rather than magic numbers.

---
**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 by reading the linked design spec and plan, then inspect src/system_ident/provenance.py and tests/test_provenance.py; .github/workflows/ci.yml:26 identifies the enforced test gate. Determine the scope for Foton ZPK/SOS export and the provenance manifest from those documents. Done means the reviewed plan is implemented without expanding SOS cascades and the provenance test gate passes.

Written by the indexing model from the issue text.