fix(trajectory): display sub-second durations as exact milliseconds

formatElapsedSeconds rounded every duration to one decimal, so calls
under 50 ms rendered as 0 s. Durations below one second now show integer
milliseconds (29 ms); at or above one second the tenth-of-a-second label
is unchanged (1.5 s). Step group descriptions reuse the same formatter.

The details-panel Duration rows toggle between the readable label and
exact milliseconds on click, mirroring the StartedAt toggle; the shared
text-selection guard is extracted for both. tabular-nums is dropped from
detail values and the toggle buttons because SF's tnum feature widens
the decimal point and leaves excessive space after it.

packages/client/ui-trajectory/tests/cell.spec.tsx

@@ -20,7 +20,10 @@ describe('formatElapsedSeconds', () => {
     expect(formatElapsedSeconds(235.0)).toBe('235 s')
     expect(formatElapsedSeconds(235.2)).toBe('235.2 s')
     expect(formatElapsedSeconds(235.25)).toBe('235.3 s')
-    expect(formatElapsedSeconds(0)).toBe('0 s')
+    expect(formatElapsedSeconds(0)).toBe('0 ms')
+    expect(formatElapsedSeconds(0.029)).toBe('29 ms')
+    expect(formatElapsedSeconds(0.5)).toBe('500 ms')
+    expect(formatElapsedSeconds(1.5)).toBe('1.5 s')
     expect(formatElapsedSeconds(Number.NaN)).toBe('—')
   })
 })

packages/client/ui-trajectory/tests/table.spec.tsx

@@ -97,6 +97,31 @@ describe('TrajectoryTable', () => {
     expect(screen.getByText('20.0 tok/s')).toBeTruthy()
   })
 
+  it('toggles a tool record Duration between readable and exact milliseconds', () => {
+    const turns: readonly TrajectoryTurnModel[] = [{
+      turn: 1,
+      groups: [{
+        title: 'Step 1',
+        cells: [{
+          index: 1,
+          kind: 'tool',
+          text: 'bash · {"command":"pwd"}',
+          inputDetail: '{"command":"pwd"}',
+          timeSeconds: 1.5,
+        }],
+      }],
+    }]
+
+    render(<TrajectoryTable turns={turns} {...FOLD_PROPS} />)
+    fireEvent.click(screen.getByRole('row', { name: /TOOL/ }))
+
+    const readable = screen.getByRole('button', { name: '1.5 s' })
+    fireEvent.click(readable)
+    expect(screen.getByRole('button', { name: '1500 ms' })).toBeTruthy()
+    fireEvent.click(screen.getByRole('button', { name: '1500 ms' }))
+    expect(screen.getByRole('button', { name: '1.5 s' })).toBeTruthy()
+  })
+
   it('breaks output tokens into labeled reasoning and content rows', () => {
     render(<TrajectoryTable turns={TURNS} {...FOLD_PROPS} />)
     fireEvent.click(screen.getByRole('row', { name: /ASSISTANT/ }))