perf(adapters): idle sources emit updates for every mousemove and scroll

[vamgan]

Summary

Idle-source adapters perform reactive state updates and notifyChanged() calls for every configured activity event, including high-frequency mousemove and scroll events.

Equivalent behavior exists across React, Vue, Svelte, Solid, and Angular. With subscribers or the inspector enabled, normal pointer movement can cause dozens or hundreds of source resolutions/serializations per second.

Proposed fix

• Update a mutable last-activity timestamp cheaply for raw activity events.
• Emit/reactively render only on meaningful active → idle and idle → active transitions, or on a bounded cadence.
• Replace the permanent five-second interval with a timeout scheduled for the idle deadline.
• Optionally expose elapsed-time updates at no more than 1 Hz.

Acceptance criteria

• 1,000 mousemove events produce bounded source-change emissions
• Idle and wake transitions occur at correct times under fake timers
• All adapters follow the same emission contract
• No persistent interval is required while activity is known
• Inspector/subscriber behavior is covered by a benchmark