v2: Warm start — reuse a view across tasks

[loyality7]

Context

Session creation is the most expensive moment in the whole lifecycle. Reusing a view across tasks where it's safe could cut that cost significantly, but must not weaken v1's isolation guarantees (fresh state per task, no bleed-through).

Scope

• Identify what's actually safe to reuse (the WebView instance vs. its state) and what must still be cleared per task.
• Measure the actual savings versus a cold session, on real hardware.
• Keep behavior opt-in if it has any observable difference from a fresh session.

Acceptance criteria

• Measured time/memory saving from reuse vs. cold start, on real hardware
• No state leaks between reused-session tasks — same 100-cycle-style acceptance gate as v1's leak test, adapted for reuse
• Default behavior unchanged unless a caller opts in