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, 53 insertions, 0 deletions

diff --git a/src/app/resolve-ws-url.test.ts b/src/app/resolve-ws-url.test.ts
new file mode 100644
index 0000000..24c2f24
--- /dev/null
+++ b/src/app/resolve-ws-url.test.ts
@@ -0,0 +1,53 @@
+import { describe, expect, it } from "vitest";
+import { resolveWsUrl } from "./resolve-ws-url";
+
+describe("resolveWsUrl", () => {
+	it("explicit url wins over everything", () => {
+		const result = resolveWsUrl(
+			{ VITE_WS_URL: "wss://env.example.com:9999" },
+			{ protocol: "https:", hostname: "page.example.com" },
+		);
+		expect(result).toBe("wss://env.example.com:9999");
+	});
+
+	it("VITE_WS_URL wins over derivation", () => {
+		const result = resolveWsUrl(
+			{ VITE_WS_URL: "wss://env.example.com:9999" },
+			{ protocol: "https:", hostname: "page.example.com" },
+		);
+		expect(result).toBe("wss://env.example.com:9999");
+	});
+
+	it("derives ws://<hostname>:24205 from http location", () => {
+		const result = resolveWsUrl({}, { protocol: "http:", hostname: "100.126.75.103" });
+		expect(result).toBe("ws://100.126.75.103:24205");
+	});
+
+	it("derives wss://<hostname>:24205 from https location", () => {
+		const result = resolveWsUrl({}, { protocol: "https:", hostname: "arch-razer" });
+		expect(result).toBe("wss://arch-razer:24205");
+	});
+
+	it("uses VITE_WS_PORT when set", () => {
+		const result = resolveWsUrl(
+			{ VITE_WS_PORT: "3000" },
+			{ protocol: "http:", hostname: "localhost" },
+		);
+		expect(result).toBe("ws://localhost:3000");
+	});
+
+	it("falls back to ws://localhost:24205 when location is missing", () => {
+		const result = resolveWsUrl({});
+		expect(result).toBe("ws://localhost:24205");
+	});
+
+	it("VITE_WS_URL empty string treated as unset", () => {
+		const result = resolveWsUrl({ VITE_WS_URL: "" }, { protocol: "http:", hostname: "myhost" });
+		expect(result).toBe("ws://myhost:24205");
+	});
+
+	it("VITE_WS_PORT empty string falls back to default", () => {
+		const result = resolveWsUrl({ VITE_WS_PORT: "" }, { protocol: "http:", hostname: "localhost" });
+		expect(result).toBe("ws://localhost:24205");
+	});
+});