fix(run): restore wheel scrolling in the terminal, suppress only arrow emulation

The previous `attachCustomWheelEventHandler(() => false)` killed scrolling
outright. xterm's `bindMouse` consults that handler in BOTH wheel paths — the
one that emits a real SGR mouse report and the fallback that turns a notch on
an alt-screen buffer into ESC[A/ESC[B — so a blanket false also blocked tmux's
own wheel scrolling (tmux runs with `mouse on`, i.e. tracking is active).

Now the handler returns false only for the exact bad case: no wheel tracking
AND an alternate buffer. `x10` counts as untracked because that protocol
reports button presses only, never the wheel, so xterm takes the emulation
path for it too.

client/src/components/run/__tests__/TerminalView.test.tsx

@@ -16,9 +16,20 @@ const openMock = vi.fn();
 const disposeMock = vi.fn();
 const wheelHandlerMock = vi.fn();
 const keyHandlerMock = vi.fn();
+// Mutable stand-ins for the two live terminal facts the wheel handler reads.
+const termState = {
+  modes: { mouseTrackingMode: "none" as "none" | "x10" | "vt200" | "drag" | "any" },
+  buffer: { active: { type: "alternate" as "normal" | "alternate" } },
+};
 
 vi.mock("@xterm/xterm", () => ({
   Terminal: vi.fn().mockImplementation(() => ({
+    get modes() {
+      return termState.modes;
+    },
+    get buffer() {
+      return termState.buffer;
+    },
     open: openMock,
     write: writeMock,
     onData: (fn: (d: string) => void) => {
@@ -58,6 +69,8 @@ global.WebSocket = MockWebSocket;
 
 describe("TerminalView", () => {
   beforeEach(() => {
+    termState.modes.mouseTrackingMode = "none";
+    termState.buffer.active.type = "alternate";
     MockWebSocket.instances = [];
     onDataHandlers.length = 0;
     writeMock.mockClear();
@@ -99,6 +112,8 @@ describe("TerminalView", () => {
   });
 
   it("swallows wheel events xterm would otherwise turn into arrow keys", () => {
+    termState.modes.mouseTrackingMode = "none";
+    termState.buffer.active.type = "alternate";
     render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);
     expect(wheelHandlerMock).toHaveBeenCalledTimes(1);
     // false = xterm skips its alt-screen wheel→cursor-key emulation, which in a
@@ -106,6 +121,29 @@ describe("TerminalView", () => {
     expect(wheelHandlerMock.mock.calls[0]![0]!(new Event("wheel"))).toBe(false);
   });
 
+  it("lets the wheel through once the pane's program tracks the mouse", () => {
+    // tmux with `mouse on` sets this; the wheel must reach it as a real SGR
+    // report, so returning false here would kill scrolling entirely.
+    termState.modes.mouseTrackingMode = "any";
+    termState.buffer.active.type = "alternate";
+    render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);
+    expect(wheelHandlerMock.mock.calls[0]![0]!(new Event("wheel"))).toBe(true);
+  });
+
+  it("lets the wheel through on a normal buffer, where xterm scrolls scrollback", () => {
+    termState.modes.mouseTrackingMode = "none";
+    termState.buffer.active.type = "normal";
+    render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);
+    expect(wheelHandlerMock.mock.calls[0]![0]!(new Event("wheel"))).toBe(true);
+  });
+
+  it("treats x10 tracking as no wheel tracking (x10 never reports the wheel)", () => {
+    termState.modes.mouseTrackingMode = "x10";
+    termState.buffer.active.type = "alternate";
+    render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);
+    expect(wheelHandlerMock.mock.calls[0]![0]!(new Event("wheel"))).toBe(false);
+  });
+
   it("keeps Shift+Tab in the terminal instead of letting focus escape", () => {
     render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);
     const handler = keyHandlerMock.mock.calls[0]![0]! as (e: KeyboardEvent) => boolean;