[wieslawsoltes]

Problem

A focused native text control keeps caret demand active. With WebGPU installed, every admitted caret frame was treated as GPU work even when the document had no JavaScript RAF, GPU canvas, or current texture. That made the worker publish unchanged scenes and could sustain a display-refresh-rate redraw loop in an otherwise idle Code OSS window.

Change

• Create a GPU rendering opportunity only when a waiting RAF may acquire a texture or an existing current texture must retire.
• Preserve the scene-lease fixture's intentional GPU work by attaching it to a real RAF.
• Add a hardware-independent CTest gate for the caret-only path.
• Exercise 10,000 synthetic 120 Hz opportunities per gate run, require zero pending GPU animation tasks after every opportunity, and cap the hot loop at two seconds.

Focused validation

• Release native engine with real V8 and pinned Dawn: built successfully.
• webscene_graphics_idle_caret_opportunity: passed.
• Focused gate: 25/25 runs, 250,000 opportunities, zero pending GPU frame tasks.
• Hot-loop timing: 0.288 ms minimum, 0.307 ms average, 0.335 ms maximum per 10,000 opportunities; ceiling is 2 seconds.
• Full webscene_graphics_v8_runtime_tests, including intentional RAF/current-texture/scene-lease behavior: passed.
• git diff --check: passed.

The previously qualified Code OSS package at WebScene consolidation b4483f3 contains the same runtime fix (00e5e90c). Its applicable idle gates recorded 0.0 visible and hidden presentations/second, 0.077% visible idle CPU, and 0.256% hidden idle CPU. The focused current-main test above makes the causal no-GPU-work condition deterministic.

Closes #114.