fix(run): keep the tmux pane exactly as wide as xterm

Scrolling a lane console repainted history with stray characters glued to
the left edge of blank lines. Reproduced against a real tmux attach: a pane
WIDER than xterm makes every over-long row autowrap its tail onto the next
row's left margin, and tmux — believing that row unchanged — never repaints
over it. One column of difference leaves one stray character per wrapped
line, two columns leave two; matched widths render clean.

TerminalView only reported its size on WS open and on the container's
ResizeObserver, so nothing corrected:
- cols moving with no container resize (webfont load, page zoom, scrollbar)
- tmux's `window-size latest` letting a second attach client (another tab,
  `ccam lanes shell`) resize the window out from under this one

Size is now reported from xterm's own onResize, re-asserted when the
terminal regains focus, and never reported as zero — the console collapses
with CSS, and fitting a 0x0 container would resize the real pane to nothing.

client 447/447, tsc clean.

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

@@ -20,7 +20,10 @@ const keyHandlerMock = vi.fn();
 const termState = {
   modes: { mouseTrackingMode: "none" as "none" | "x10" | "vt200" | "drag" | "any" },
   buffer: { active: { type: "alternate" as "normal" | "alternate" } },
+  cols: 131,
+  rows: 28,
 };
+const onResizeHandlers: Array<() => void> = [];
 
 vi.mock("@xterm/xterm", () => ({
   Terminal: vi.fn().mockImplementation(() => ({
@@ -30,6 +33,16 @@ vi.mock("@xterm/xterm", () => ({
     get buffer() {
       return termState.buffer;
     },
+    get cols() {
+      return termState.cols;
+    },
+    get rows() {
+      return termState.rows;
+    },
+    onResize: (fn: () => void) => {
+      onResizeHandlers.push(fn);
+      return { dispose: vi.fn() };
+    },
     open: openMock,
     write: writeMock,
     onData: (fn: (d: string) => void) => {
@@ -53,6 +66,8 @@ class MockWebSocket {
   onopen: (() => void) | null = null;
   onmessage: ((e: { data: unknown }) => void) | null = null;
   onclose: (() => void) | null = null;
+  readyState = 1; // OPEN — the component only sends on an open socket
+  static OPEN = 1;
   constructor(url: string) {
     this.url = url;
     MockWebSocket.instances.push(this);
@@ -71,8 +86,11 @@ describe("TerminalView", () => {
   beforeEach(() => {
     termState.modes.mouseTrackingMode = "none";
     termState.buffer.active.type = "alternate";
+    termState.cols = 131;
+    termState.rows = 28;
     MockWebSocket.instances = [];
     onDataHandlers.length = 0;
+    onResizeHandlers.length = 0;
     writeMock.mockClear();
     wheelHandlerMock.mockClear();
     keyHandlerMock.mockClear();
@@ -161,6 +179,43 @@ describe("TerminalView", () => {
     expect(preventPlainTab).not.toHaveBeenCalled();
   });
 
+  describe("size reporting (tmux must never be wider than the terminal)", () => {
+    // A tmux pane wider than xterm wraps each over-long row's tail onto the
+    // next row's left margin, and tmux never repaints a row it believes
+    // unchanged — the stray characters glued to the left edge after scrolling.
+    it("reports its size when the socket opens", () => {
+      render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);
+      const ws = MockWebSocket.instances[0]!;
+      ws.onopen?.();
+      expect(ws.sent).toContain(JSON.stringify({ type: "resize", cols: 131, rows: 28 }));
+    });
+
+    it("reports a size change xterm made on its own, with no container resize", () => {
+      // Webfont load and page zoom both move cols without resizing the element
+      // ResizeObserver watches, which is why onResize is the hook.
+      render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);
+      const ws = MockWebSocket.instances[0]!;
+      ws.onopen?.();
+      ws.sent.length = 0;
+      termState.cols = 129;
+      onResizeHandlers.forEach((fn) => fn());
+      expect(ws.sent).toEqual([JSON.stringify({ type: "resize", cols: 129, rows: 28 })]);
+    });
+
+    it("never reports a collapsed console's zero size", () => {
+      // The Workspace console collapses with CSS, so the container can be 0×0
+      // while mounted; resizing the real tmux pane to that would wreck it.
+      render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);
+      const ws = MockWebSocket.instances[0]!;
+      ws.sent.length = 0;
+      termState.cols = 0;
+      termState.rows = 0;
+      onResizeHandlers.forEach((fn) => fn());
+      ws.onopen?.();
+      expect(ws.sent).toEqual([]);
+    });
+  });
+
   it("re-attaches after the socket drops (a server restart must not freeze the pane)", () => {
     vi.useFakeTimers();
     render(<TerminalView runId="ccam-lane-1" wsBaseUrl="ws://localhost:4820" />);