Add optional in-place reset mode to pool

[moperacz]

Summary

• add PoolResetMode so hosts can choose between full teardown and in-place reuse
• teach language engines/JsRuntime how to reset without dropping the isolate
• surface reset telemetry (mode, duration, engine generation) via diagnostics/telemetry for both manual and pooled resets
• bake overlays into warm snapshots so in-place resets skip the expensive overlay import, with tests/docs explaining the fast path

Testing

• cargo test --workspace --exclude integration-tests