docs(CONTRACT): document valve's lack of a maintenance state machine

valve's schema mode enum includes `maintenance` (which gates sources)
but the FSM has no `entermaintenance` / `exitmaintenance` states and
the schema's `sequences` block has only startup / shutdown /
emergencystop / boot. Maintenance mode therefore disables external
sources but doesn't run a maintenance sequence — different shape
from rotatingMachine. Added a Limitations section to the CONTRACT so
this is explicit rather than surfacing as a wiki TODO.

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

CONTRACT.md

@@ -68,6 +68,18 @@ from the measurement container (e.g. `delta_predicted_pressure`,
   (`idle → starting → warmingup → operational → accelerating →
   decelerating → stopping → coolingdown → idle`, plus `off`).
 
+## Limitations vs `rotatingMachine`
+
+- **No `maintenance` state machine.** The schema's `mode.current` enum
+  accepts `maintenance` (gates sources via `isValidSourceForMode`), but
+  the FSM has no `entermaintenance` / `exitmaintenance` states and the
+  `sequences` schema declares only `startup`, `shutdown`, `emergencystop`,
+  and `boot`. Configuring `maintenance` mode therefore disables external
+  command sources but does not put the valve through a maintenance
+  sequence. Aligns with valve's role as a passive flow-controlled
+  actuator; lift to RM-style FSM if/when site maintenance procedures
+  require explicit state transitions.
+
 ## Events emitted by `source.measurements.emitter`
 
 The `MeasurementContainer` fires `<type>.<variant>.<position>` whenever