fix(js): end browser trace roots at real activity, not idle-timeout padding

[DriesH]

Stacked on #69 — targets research/tracing-framework-routers, so the diff here is just this fix.

Summary

Browser pageload / navigation root spans were reporting durations inflated by the whole idleTimeout. A childless root (no fetch/xhr children during its window) closed at now() = start + idleTimeout, so a real ~488ms pageload showed as 2488ms with idleTimeout: 2000.

Root cause

IdleRootController trims a root's end to the last child's end on idle close, but fell back to now() when there were no children (lastChildEndTime ?? now()), padding the duration by the full idle window. endNow() (route change / pagehide) had the same now() behavior.

Fix

Roots now trim to an injected end floor instead of now():

• pageload → Navigation Timing loadEventEnd (the browser's "page finished loading" mark; falls back to domContentLoadedEventEnd, then now()).
• navigation → the nav's own start (an instant client-side nav trims to ~0).
• endNow() trims the same way, except when a child is still in flight — then it keeps now() so in-progress work isn't cut short.

Fetch-heavy roots are unchanged; they already trimmed to the last child.

Tests

• IdleRootController: childless roots trim to the floor (not now()); endNow trims unless a child is open.
• navigationTiming: computePageloadEndNano (loadEventEnd → domContentLoadedEventEnd → now fallbacks).
• Full @flareapp/js suite green (154), typecheck + lint clean.

Verified in the JS playground against a local Flare: the pageload that read 2488ms now reports its real ~488ms.