fix: the API emitted undefined optionals as null, and the UI couldn't decode them (v0.3.2)

`Schema.optional(X)` means `X | undefined`, and the plain Schema encoder omits a
present-but-undefined key — but the HttpApi RESPONSE path serialises it as
`null`, which `X | undefined` then refuses when the client decodes it. Two
user-visible failures shipped in 0.3.1:

  - GET /api/status on a never-synced host returned `"lastSync":null`, so the
    Overview page could not decode its own status — i.e. every fresh install
    until the first successful sync.
  - POST /api/detect on a box with any emulator installed returned
    `"contested":null` / `"appId":null`, so the Emulators page's Detect button
    could not decode the result. It only looked fine because a value that has
    round-tripped through cache.json has those keys ABSENT rather than present
    -and-undefined, and absent keys encode correctly.

Nothing on this wire is `undefined` any more. EngineStatus.{lastSync,lastReport,
detectedAt} are `Schema.NullOr` and the engine sends explicit nulls. The
DetectedEmulator fields use `Schema.optional(Schema.NullOr(...))` instead —
tolerant of absent, undefined and null — because they also live in cache.json,
and requiring them would fail the whole cache decode on upgrade and silently
discard every art verdict.

plugin/test/wire.test.ts pins it: it drives the real makeApi handler stack
through toWebHandler and decodes the bytes with the shared contract schemas.
Against the old schema all five of its assertions fail; a unit test of either
side in isolation passes while the product is broken.

Also: the SSE feed now emits the current status on subscribe rather than only on
the next engine transition, so a freshly-opened console page is correct
immediately instead of stale until something happens.

ui/src/data/atoms.ts

@@ -120,7 +120,7 @@ export const statusEventsAtom: Atom.Atom<ReadonlyArray<StatusEvent>> =
 					if (!prev.syncing && frame.syncing) push("Sync started");
 					if (prev.syncing && !frame.syncing) {
 						const report = frame.lastReport;
-						if (report === undefined) {
+						if (report === null) {
 							push("Sync finished");
 						} else {
 							push(
@@ -136,7 +136,7 @@ export const statusEventsAtom: Atom.Atom<ReadonlyArray<StatusEvent>> =
 						}
 					}
 					if (
-						frame.detectedAt !== undefined &&
+						frame.detectedAt !== null &&
 						frame.detectedAt !== prev.detectedAt
 					) {
 						push("Emulator detection finished");

ui/src/mocks/fixtures.ts

@@ -295,11 +295,16 @@ export const statusFor = (scenario: Scenario): EngineStatus => {
 	const report = reportFor(scenario, configFor(scenario));
 	switch (scenario) {
 		case "firstRun":
+			// Explicit nulls, exactly as the server sends them — a fixture that omitted
+			// these would no longer resemble the real wire shape.
 			return {
 				rootsConfigured: 0,
 				os: "linux",
 				artProvider: null,
 				syncing: false,
+				lastSync: null,
+				lastReport: null,
+				detectedAt: null,
 				paths: PATHS,
 			};
 		case "syncing":

ui/src/pages/emulators.tsx

@@ -174,14 +174,13 @@ const DetectedList = ({
 										contested
 									</Badge>
 								) : null}
-								{row.detected?.cores !== undefined &&
-								row.detected.cores.length > 0 ? (
+								{(row.detected?.cores?.length ?? 0) > 0 ? (
 									<Badge variant="outline" size="sm">
-										{row.detected.cores.length} cores
+										{row.detected?.cores?.length} cores
 									</Badge>
 								) : null}
 							</div>
-							{row.detected?.exePath !== undefined ? (
+							{row.detected?.exePath ? (
 								<p className="mt-0.5 truncate font-mono text-xs text-muted-foreground">
 									{row.detected.exePath}
 								</p>

ui/src/pages/overview.tsx

@@ -72,7 +72,7 @@ const FirstRun = ({ navigate }: PageProps) => (
 );
 
 const lastSyncHint = (status: EngineStatus): string => {
-	if (status.lastSync === undefined) return "never synced";
+	if (status.lastSync === null) return "never synced";
 	const minutes = Math.max(
 		0,
 		Math.round((Date.now() - status.lastSync.at) / 60_000),
@@ -117,7 +117,7 @@ const SyncButton = ({ status }: { status: EngineStatus }) => {
 
 const Warnings = ({ status }: { status: EngineStatus }) => {
 	const report = status.lastReport;
-	if (report === undefined) return null;
+	if (report === null) return null;
 	const count = report.warnings.length + (report.overWarn ? 1 : 0);
 	if (count === 0) return null;
 	return (