P6: convert diffuser to BaseDomain + BaseNodeAdapter + concern split

Refactor of diffuser to use the platform infrastructure (BaseDomain, BaseNodeAdapter,
ChildRouter, commandRegistry, statusBadge). Extracts concerns into
focused modules per .claude/refactor/MODULE_SPLIT.md generic template.
Tests stay green; CONTRACT.md generated; legacy aliases preserved.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CONTRACT.md

@@ -0,0 +1,71 @@
+# diffuser — Contract
+
+Hand-maintained for Phase 6; the `## Inputs` table is generated from
+`src/commands/index.js` (see Phase 9 generator). Keep ≤ 100 lines.
+
+## Inputs (msg.topic on Port 0)
+
+| Canonical | Aliases (deprecated) | Payload | Effect |
+|---|---|---|---|
+| `data.flow` | `air_flow` | `number` — airflow in Nm³/h | Calls `source.setFlow(payload)`; clamps to ≥ 0 and recomputes OTR. |
+| `set.density` | `density` | `number` — diffuser density (per m²) | Calls `source.setDensity(payload)` and recomputes. |
+| `set.water-height` | `height_water` | `number` — water column height in m | Calls `source.setWaterHeight(payload)`; clamps to ≥ 0 and recomputes head + total pressure. |
+| `set.header-pressure` | `header_pressure` | `number` — header gauge pressure in mbar | Calls `source.setHeaderPressure(payload)` and recomputes. |
+| `set.elements` | `elements` | `number` — element count (rounded; must be > 0) | Calls `source.setElementCount(payload)` and recomputes per-element flow. |
+| `set.alfa-factor` | `alfaFactor` | `number` — alpha correction (≥ 0) | Calls `source.setAlfaFactor(payload)` and recomputes oxygen output. |
+
+Aliases log a one-time deprecation warning the first time they fire.
+
+## Outputs (msg.topic on Port 0/1/2)
+
+- **Port 0 (process):** `msg.topic = config.general.name`. Payload built
+  by `outputUtils.formatMsg(..., 'process')` from `getOutput()` —
+  delta-compressed (only changed fields are emitted). Fields:
+  - `iPressure`, `iMWater`, `iFlow` — echoed inputs.
+  - `nFlow` — normalised airflow (Nm³/h).
+  - `oOtr` — interpolated oxygen transfer rate (g O₂ / Nm³).
+  - `oPLoss` — total head loss (mbar) = static head + diffuser ΔP.
+  - `oKgo2H` — kg O₂ per hour at current operating point.
+  - `oFlowElement` — flow per element (Nm³/h/element).
+  - `efficiency` — combined OTR/ΔP efficiency (0–100).
+  - `slope` — local OTR-vs-flow slope.
+  - `oZoneOtr` — reactor zone OTR (kg O₂ / m³ / day) computed against
+    `diffuser.zoneVolume`; `0` when zone volume is unset.
+  - `idle` — true when `data.flow ≤ 0`.
+  - `warning`, `alarm` — string arrays describing flow-per-element band
+    excursions.
+- **Port 1 (InfluxDB telemetry):** same shape as Port 0, formatted with
+  the `'influxdb'` formatter.
+- **Port 2 (registration):** at startup the node sends one
+  `{ topic: 'child.register', payload: <node.id>, positionVsParent,
+  distance }` to the upstream parent (typically a reactor).
+  `positionVsParent` defaults to `'atEquipment'`.
+
+## Port-count change (Phase 6)
+
+Pre-refactor the diffuser exposed 4 outputs (process, dbase, reactor
+control with `topic: 'OTR'`, parent registration). The reactor control
+message merged into Port 0 as `oZoneOtr`; consumers that previously
+listened to the dedicated control port should switch to reading
+`payload.oZoneOtr` from the process output. The legacy `OTR` topic is
+removed in this refactor — there is no alias, since the data shape
+differs (single value vs full process payload).
+
+## Events emitted by `source.measurements.emitter`
+
+None today. The diffuser does not currently publish typed measurements
+through `MeasurementContainer`; all output flows via `getOutput()`.
+A future phase may promote `oOtr` and `oZoneOtr` to typed series so
+parent reactors can subscribe through the standard `ChildRouter`
+handshake.
+
+## Events emitted by `source.emitter`
+
+- `output-changed` — fires whenever an input setter recomputes the
+  oxygen-transfer state. `BaseNodeAdapter` listens and pushes the
+  delta-compressed Port 0 / Port 1 messages.
+
+## Children registered by this node
+
+None. The diffuser is a leaf Equipment Module; it registers itself with
+its parent (reactor / process cell) via the Port 2 handshake.