fix: adaptive detection uses real speed probe — works with DevTools throttling

Critical fix: navigator.connection API does NOT reflect DevTools
network throttling. It reports the underlying physical connection,
not the simulated speed.

New approach:
1. Synchronous initial detection (navigator.connection + TTFB) — instant
2. Async speed probe: fetches a small resource and measures actual throughput
3. If measured speed is slower than API-reported, downgrades tier
4. Only downgrades, never upgrades — prevents premium→lite flicker

Test page also updated with speed probe approach.
Classification: ≥2Mbps→premium, 0.5-2→standard, 0.1-0.5→standard(motion 1), <0.1→lite

Test: throttle to GPRS in DevTools → reload → should now correctly detect lite.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: annondeveloper