Add position constants, reactor/settler config schemas

- New src/constants/positions.js: POSITIONS enum (upstream/downstream/atEquipment/delta)
- New src/configs/reactor.json: Full schema for CSTR/PFR reactor parameters and ASM3 initial state
- New src/configs/settler.json: Schema for settler node
- Export POSITIONS, POSITION_VALUES, isValidPosition from index.js

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/constants/positions.js

@@ -0,0 +1,18 @@
+/**
+ * Canonical position constants for parent-child relationships.
+ * Use these instead of hardcoded strings throughout the codebase.
+ */
+const POSITIONS = Object.freeze({
+  UPSTREAM: 'upstream',
+  DOWNSTREAM: 'downstream',
+  AT_EQUIPMENT: 'atEquipment',
+  DELTA: 'delta',
+});
+
+const POSITION_VALUES = Object.freeze(Object.values(POSITIONS));
+
+function isValidPosition(pos) {
+  return POSITION_VALUES.includes(pos);
+}
+
+module.exports = { POSITIONS, POSITION_VALUES, isValidPosition };