docs: Umber concentrator deployment; add Fi-Wi L4S spec to repo

- README: where FiWiControl runs (Umber concentrator), link spec path
- README: repo tree includes html/Fi-Wi-L4S.php
- Add html/Fi-Wi-L4S.php (architecture / L4S specification page)

Made-with: Cursor

README.md

@@ -2,6 +2,8 @@
 
 Tools, libraries and statistical software for automating, managing, monitoring and testing Umber Fi‑Wi networks.
 
+**Where it runs:** FiWiControl is built to run **on the Umber concentrator** — the Fi‑Wi control plane described in the architecture spec (**`html/Fi-Wi-L4S.php`** in this repo) — for **lab and customer** automation. Day-to-day development still uses **workstation** installs and **lab rigs** (e.g. Raspberry Pi) as in **`docs/install.md`**.
+
 **Naming:** The **Git repository** and checkout directory are **FiWiControl** (mixed case). The **Python distribution** and **import package** are **`fiwicontrol`** (all lowercase, PEP 8) — same project, different casing rules for Git vs Python. Use **`fiwicontrol`** for `pip install` / `import`, not `FiWiControl`.
 
 This repository ships that distribution (**`fiwicontrol`** on PyPI / `pip`) with import root **`fiwicontrol`**:
@@ -47,6 +49,8 @@ FiWiControl/
 ├── LICENSE
 ├── README.md
 ├── pyproject.toml
+├── html/
+│   └── Fi-Wi-L4S.php
 ├── docs/
 │   ├── install.md
 │   ├── node-control-asyncio-design.md