fix(canvas/Tooltip): make aria-describedby conditional (show ? id : undefined)

Adopts PR #299's WCAG-correct approach. aria-describedby must only
reference content that exists in the DOM — setting it unconditionally
points to a non-existent ID when the tooltip portal is not mounted,
producing undefined browser/AT behavior.

Changes:
- Tooltip.tsx: aria-describedby={show ? tooltipId.current : undefined}
- Tooltip.test.tsx: 3 new aria-describedby tests:
  1. does NOT set aria-describedby when tooltip is hidden
  2. sets aria-describedby when tooltip shown (hover)
  3. sets aria-describedby when tooltip shown (keyboard focus)

Also fixes PR #306 Tooltip test which asserted unconditional aria-describedby
— this would have failed under PR #299's conditional approach.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

canvas/src/components/Tooltip.tsx

@@ -77,7 +77,10 @@ export function Tooltip({ text, children }: Props) {
       onMouseLeave={leave}
       onFocus={onFocus}
       onBlur={onBlur}
-      aria-describedby={tooltipId.current}
+      // WCAG / ARIA: only set aria-describedby when the tooltip portal is
+      // actually mounted in the DOM. This keeps the referenced ID always valid
+      // and avoids dangling aria-describedby pointing to a non-existent element.
+      aria-describedby={show ? tooltipId.current : undefined}
     >
       {children}
       {show && text && createPortal(

canvas/src/components/__tests__/Tooltip.test.tsx

@@ -221,23 +221,64 @@ describe("Tooltip — Esc dismiss (WCAG 1.4.13)", () => {
   });
 });
 
-describe("Tooltip — aria-describedby", () => {
+describe("Tooltip — aria-describedby (WCAG / ARIA best practice)", () => {
   beforeEach(() => { vi.useFakeTimers(); });
   afterEach(() => { vi.useRealTimers(); });
 
-  it("associates tooltip with the trigger via aria-describedby", () => {
+  it("does NOT set aria-describedby when tooltip is hidden (WCAG 1.4.13)", () => {
+    // aria-describedby must only reference content that exists in the DOM.
+    // Setting it unconditionally points to a non-existent ID when the portal
+    // is not mounted — undefined browser/AT behavior.
     const { container } = render(
-      <Tooltip text="Associated tip">
+      <Tooltip text="Hidden tip">
         <button type="button">Hover me</button>
       </Tooltip>
     );
+    // Without any hover/focus, the tooltip is not shown
     const wrapper = container.querySelector("[aria-describedby]");
-    const describedBy = wrapper?.getAttribute("aria-describedby");
-    expect(describedBy).toBeTruthy();
-    // Show the tooltip first so the portal element is in the DOM
+    expect(wrapper).toBeNull();
+  });
+
+  it("sets aria-describedby when tooltip is shown (hover)", () => {
+    const { container } = render(
+      <Tooltip text="Shown tip">
+        <button type="button">Hover me</button>
+      </Tooltip>
+    );
+    // Before hover: no aria-describedby
+    const wrapperBefore = container.querySelector("[aria-describedby]");
+    expect(wrapperBefore).toBeNull();
+
+    // Hover → tooltip shows
     fireEvent.mouseEnter(container.querySelector("button")!);
     act(() => { vi.advanceTimersByTime(500); });
-    // The describedby id must now resolve to the tooltip portal element
+
+    // After hover: aria-describedby is set and references the portal element
+    const wrapperAfter = container.querySelector("[aria-describedby]");
+    const describedBy = wrapperAfter?.getAttribute("aria-describedby");
+    expect(describedBy).toBeTruthy();
+    expect(document.getElementById(describedBy!)).toBeTruthy();
+  });
+
+  it("sets aria-describedby when tooltip is shown (keyboard focus)", () => {
+    const { container } = render(
+      <Tooltip text="Focus tip">
+        <button type="button">Focus me</button>
+      </Tooltip>
+    );
+    // Before focus: no aria-describedby
+    const wrapperBefore = container.querySelector("[aria-describedby]");
+    expect(wrapperBefore).toBeNull();
+
+    // Focus → tooltip shows immediately (no timer)
+    act(() => {
+      container.querySelector("button")!.focus();
+    });
+
+    // After focus: aria-describedby is set
+    const wrapperAfter = container.querySelector("[aria-describedby]");
+    const describedBy = wrapperAfter?.getAttribute("aria-describedby");
+    expect(describedBy).toBeTruthy();
     expect(document.getElementById(describedBy!)).toBeTruthy();
   });
 });