Slice 1: surface system + WS transport + composition root

Pure-core feature libraries assembled at the composition root:
- core/protocol: pure reducer over surface catalog/spec/error messages
- features/surface-host: generic field-kind interpreter (toggle/progress/
  selector/stat/button) + pure plan logic; no surface-id special-casing
- adapters/ws: injected WebSocket client (effects at the edge)
- app: composition root store (Svelte 5 runes over the pure reducer),
  host-relative surface WS URL resolution (resolveWsUrl), root App.svelte

Verified green: svelte-check 0/0, vitest 84 passed, biome clean, vite build ok.

1 files changed, 74 insertions, 0 deletions

diff --git a/src/features/surface-host/logic/plan.ts b/src/features/surface-host/logic/plan.ts
new file mode 100644
index 0000000..5b4530b
--- /dev/null
+++ b/src/features/surface-host/logic/plan.ts
@@ -0,0 +1,74 @@
+import type { InvokeMessage, SurfaceSpec } from "@dispatch/ui-contract";
+import type { FieldView, SurfaceRenderPlan } from "./types";
+
+const KNOWN_KINDS = new Set(["toggle", "progress", "selector", "stat", "button"]);
+
+/**
+ * Validate and normalise a SurfaceSpec into a renderable plan.
+ * Keeps known field kinds in order; drops unknown kinds and `custom` fields
+ * (no renderer registry yet — graceful skip, never throw).
+ */
+export function planSurface(spec: SurfaceSpec): SurfaceRenderPlan {
+	const fields: FieldView[] = [];
+	for (const field of spec.fields) {
+		if (!KNOWN_KINDS.has(field.kind)) continue;
+		switch (field.kind) {
+			case "toggle":
+				fields.push({
+					kind: "toggle",
+					label: field.label,
+					value: field.value,
+					action: field.action,
+				});
+				break;
+			case "progress":
+				fields.push({
+					kind: "progress",
+					label: field.label,
+					value: field.value,
+				});
+				break;
+			case "selector":
+				fields.push({
+					kind: "selector",
+					label: field.label,
+					value: field.value,
+					options: field.options,
+					action: field.action,
+				});
+				break;
+			case "stat":
+				fields.push({
+					kind: "stat",
+					label: field.label,
+					value: field.value,
+				});
+				break;
+			case "button":
+				fields.push({
+					kind: "button",
+					label: field.label,
+					action: field.action,
+				});
+				break;
+		}
+	}
+	return { fields };
+}
+
+/**
+ * Construct a typed `invoke` client message for an actionable field.
+ * For toggle the payload is the new boolean; for selector the chosen value;
+ * for button the payload is omitted.
+ */
+export function buildInvoke(
+	surfaceId: string,
+	field: Extract<FieldView, { action: unknown }>,
+	value?: unknown,
+): InvokeMessage {
+	const base = { type: "invoke" as const, surfaceId, actionId: field.action.actionId };
+	if (value !== undefined) {
+		return { ...base, payload: value };
+	}
+	return base;
+}