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/src/client/layout.ts

@@ -17,6 +17,7 @@ import type {
   TrajectoryCellProps,
   TrajectorySourceBlock,
 } from './trajectory-record.ts'
+import { formatElapsedSeconds } from './trajectory-record.ts'
 
 /** One Message or Step group inside a turn. */
 export interface TrajectoryGroupModel {
@@ -603,9 +604,7 @@ function groupDescription(laid: readonly LaidCell[]): string | undefined {
 
 function formatGroupDuration(seconds: number): string | undefined {
   if (!Number.isFinite(seconds)) return undefined
-  const rounded = Math.round(seconds * 10) / 10
-  if (Number.isInteger(rounded)) return `${rounded} s`
-  return `${rounded.toFixed(1)} s`
+  return formatElapsedSeconds(seconds)
 }
 
 /** Own-duration seconds from two epoch-ms stamps; null when either is unusable. */