v2: Device emulation — viewport, scale, touch, orientation, locale, timezone

[loyality7]

Context

Some of this is available through CDP (Emulation.setDeviceMetricsOverride was confirmed present in the v1 probe), some only through platform APIs. The unified API should hide which backend serves which, same pattern as everything else in v1.

Scope

• Viewport size and device scale factor at runtime (v1 only sets this at session creation).
• Touch emulation, orientation, locale, timezone override.
• capabilities() reports what's actually available on this device/backend combination.

Acceptance criteria

• Viewport/scale factor changeable after a session is already open
• Touch/orientation/locale/timezone emulation each has a device test proving the page actually observes the emulated value
• Absent capabilities fail fast with UNSUPPORTED, never silently no-op