fix(observatory): bound the concurrency cap and clarify the lower control

Fold #1418 kilo review:
- The lower button no longer doubles as a clear: it floors at 1 and disables
  there, so its 'Lower' label is never misleading. Removal stays the explicit
  Clear button.
- Clamp the cap to a ceiling (MAX_CAP 50) and disable Raise at the top, so the
  stepper cannot post runaway values; coerceCap also clamps an out-of-range
  server value.
Tests updated for the Clear-button path and the new ceiling.

Author: jaylfc

desktop/src/apps/ObservatoryApp.test.tsx

@@ -135,7 +135,7 @@ describe("ObservatoryApp", () => {
     });
   });
 
-  it("clears the cap to null from the lowest step", async () => {
+  it("clears the cap to null via the Clear button", async () => {
     const fetchMock = mockFetch({
       "GET /api/observatory/fleet": { ok: true, body: fleetBody },
       "GET /api/observatory/throttle": { ok: true, body: { global: 1, lanes: {} } },
@@ -148,7 +148,12 @@ describe("ObservatoryApp", () => {
       expect(screen.getByLabelText(/concurrency cap value/i).textContent).toBe("1"),
     );
 
-    fireEvent.click(screen.getByRole("button", { name: /lower concurrency cap/i }));
+    // The lower button floors at 1 (disabled there); removal is the explicit Clear.
+    expect(
+      (screen.getByRole("button", { name: /lower concurrency cap/i }) as HTMLButtonElement)
+        .disabled,
+    ).toBe(true);
+    fireEvent.click(screen.getByRole("button", { name: /^clear$/i }));
     await flush();
 
     const post = fetchMock.mock.calls.find(
@@ -160,6 +165,25 @@ describe("ObservatoryApp", () => {
     });
   });
 
+  it("disables raising the cap at the ceiling", async () => {
+    vi.stubGlobal(
+      "fetch",
+      mockFetch({
+        "GET /api/observatory/fleet": { ok: true, body: fleetBody },
+        "GET /api/observatory/throttle": { ok: true, body: { global: 50, lanes: {} } },
+      }),
+    );
+    render(<ObservatoryApp windowId="w1" />);
+    await flush();
+    await waitFor(() =>
+      expect(screen.getByLabelText(/concurrency cap value/i).textContent).toBe("50"),
+    );
+    expect(
+      (screen.getByRole("button", { name: /raise concurrency cap/i }) as HTMLButtonElement)
+        .disabled,
+    ).toBe(true);
+  });
+
   it("shows the idle empty state when no agents are working", async () => {
     vi.stubGlobal(
       "fetch",

desktop/src/apps/ObservatoryApp.tsx

@@ -24,8 +24,10 @@ const EMPTY_PAUSE: PauseState = { global: false, lanes: {} };
 // Global concurrency cap: how many cards the fleet may hold in flight at once
 // (the dispatch loop reads it as MAX_OPEN_PRS). null = no override, the loop
 // default applies. Pause is the on/off switch; this is the volume knob.
+const MAX_CAP = 50; // sane ceiling so the stepper cannot post runaway values
 function coerceCap(v: unknown): number | null {
-  return typeof v === "number" && Number.isFinite(v) && v > 0 ? Math.floor(v) : null;
+  if (typeof v !== "number" || !Number.isFinite(v) || v <= 0) return null;
+  return Math.min(MAX_CAP, Math.floor(v));
 }
 
 export function ObservatoryApp({ windowId: _windowId }: { windowId: string }) {
@@ -156,8 +158,8 @@ export function ObservatoryApp({ windowId: _windowId }: { windowId: string }) {
         <div className="flex items-center gap-1.5">
           <button
             type="button"
-            onClick={() => setGlobalCap(cap && cap > 1 ? cap - 1 : null)}
-            disabled={busy === "cap" || cap == null}
+            onClick={() => cap != null && cap > 1 && setGlobalCap(cap - 1)}
+            disabled={busy === "cap" || cap == null || cap <= 1}
             aria-label="Lower concurrency cap"
             className="flex h-7 w-7 items-center justify-center rounded-md border border-shell-border text-shell-text-secondary transition-colors hover:text-shell-text hover:border-shell-border-strong disabled:cursor-not-allowed disabled:opacity-40"
           >
@@ -171,8 +173,8 @@ export function ObservatoryApp({ windowId: _windowId }: { windowId: string }) {
           </span>
           <button
             type="button"
-            onClick={() => setGlobalCap((cap ?? 0) + 1)}
-            disabled={busy === "cap"}
+            onClick={() => setGlobalCap(Math.min(MAX_CAP, (cap ?? 0) + 1))}
+            disabled={busy === "cap" || (cap != null && cap >= MAX_CAP)}
             aria-label="Raise concurrency cap"
             className="flex h-7 w-7 items-center justify-center rounded-md border border-shell-border text-shell-text-secondary transition-colors hover:text-shell-text hover:border-shell-border-strong disabled:cursor-not-allowed disabled:opacity-40"
           >