docs: add CLAUDE.md for SLMM

Split out of the shared ~/CLAUDE.md, which had drifted badly — it described
SLMM at v0.1.0/v0.2.1 with none of the v0.4.0 design in it.

Written from the changelog and the actual tree. Leads with the three things
that define the current architecture: the fan-out monitor (the NL-43 has ONE
TCP control connection, which is the constraint the whole design works
around), the alert engine that pins the monitor on so rules evaluate 24/7,
and the nl43_readings history trail for live-chart backfill.

Defers to CHANGELOG.md for history rather than duplicating a version list.

Stack-level context lives in ../terra-view/docs/tmi-stack.md, loaded as
~/CLAUDE.md.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HgTe8CamXAHcAmaQ6QNcog

CLAUDE.md

@@ -0,0 +1,91 @@
+# CLAUDE.md — SLMM (Sound Level Meter Manager)
+
+Device module for **Rion NL-43 / NL-53** sound level meters. Translates
+Terra-View's REST calls into the meter's ASCII command protocol over TCP, and
+pulls stored data over FTP. Current version: **v0.4.0**.
+
+Stack-level context (which repo owns what, version pairing) lives in
+`../terra-view/docs/tmi-stack.md`, which is also loaded as `~/CLAUDE.md`.
+
+**Terra-View never talks to a meter directly** — it always goes through here.
+SLMM owns the protocol; Terra-View owns the UI and the measurement records.
+
+---
+
+## Where things stand (updated 2026-08-29)
+
+v0.4.0 (2026-06-22) is the current release. The three things that define the
+current design:
+
+- **Fan-out live monitor.** The NL-43 has **one** TCP control connection, and
+  every dashboard used to fight for it. Now a single poller reads the device and
+  all subscribers share one cached feed: `WS /api/nl43/{unit_id}/monitor`
+  delivers an instant first frame from cache, then live updates. Poll rate
+  adapts to demand, unreachable devices back off, and the background poller
+  skips units already covered by an active monitor so nothing double-polls.
+- **Alert engine.** Per-device threshold rules (metric + threshold + cooldown)
+  with full CRUD, an onset/clear state machine, and acknowledgement. Enabled
+  rules **pin the monitor on**, so they evaluate 24/7 with no UI client
+  connected. Editing or deleting a rule resets its state and closes any open
+  event.
+- **History for chart backfill.** A downsampled trail persists to `nl43_readings`
+  and is served from `GET /api/nl43/{unit_id}/history`, so live charts can fill
+  in recent history on load. L1/L10 percentiles are surfaced in status and the
+  live feed.
+
+`CHANGELOG.md` is the authority for anything older; prefer it over prose here.
+
+---
+
+## Layout
+
+```
+app/
+  main.py              FastAPI app + health checks
+  routers.py           all REST endpoints
+  services.py          NL43Client — TCP + FTP protocol
+  monitor.py           the fan-out live feed
+  background_poller.py scheduled polling
+  alerts.py            rule evaluation + event state machine
+  models.py            NL43Config, NL43Status, nl43_readings
+  database.py          SQLAlchemy / SQLite (data/slmm.db)
+  device_logger.py
+docs/     manuals/     templates/     nl52/     SLM-stress-test/
+```
+
+Key docs: `docs/API.md` (endpoint reference with curl examples),
+`docs/COMMUNICATION_GUIDE.md` and `docs/nl43_Command_ref.md` (protocol).
+
+---
+
+## API surface
+
+```
+GET/PUT /api/nl43/{unit}/config
+GET     /api/nl43/{unit}/status            cached snapshot
+GET     /api/nl43/{unit}/live              fresh read
+POST    /api/nl43/{unit}/{start|stop|pause|resume|reset|store}
+GET     /api/nl43/{unit}/{battery|clock|results|settings}
+WS      /api/nl43/{unit}/monitor           fan-out live feed
+POST    /api/nl43/{unit}/monitor/{start|stop}
+GET     /api/nl43/_monitor/status
+GET     /api/nl43/{unit}/history           backfill trail
+        .../alerts/rules  .../alerts/events  .../events/{id}/ack
+POST/GET /api/nl43/{unit}/ftp/{enable|disable|status|files|download}
+GET     /api/nl43/{unit}/overwrite-check   BEFORE changing a store name
+```
+
+---
+
+## Gotchas
+
+- **One TCP connection per meter.** This is the constraint the whole monitor
+  design exists to work around. Never add a code path that opens its own
+  connection alongside the monitor.
+- **1-second minimum between commands.** The NL-43 protocol requires it; SLMM
+  enforces it automatically. Do not "optimise" it away.
+- **Check `overwrite-check` before changing a store name** — otherwise you can
+  destroy data on the meter's SD card.
+- **FTP is active mode** — the device connects *back* to the server on port 21.
+- **`network_mode: host`** in compose, for direct network access to meters.
+- Environment: `PORT` (8100), `CORS_ORIGINS`, `TIMEZONE_OFFSET`, `TIMEZONE_NAME`.