debugSamplerCount property

Every GPU object this device owns and would have to release: the textures and geometry it handed out, the frame arenas and fallbacks it made for itself, and the modules, layouts, samplers, bind groups and pipelines it cached.

Not zero on a fresh device, unlike the WebGL2 backend's count of the same name, and the difference is a fact about the two APIs rather than about the two implementations: this one allocates three arena buffers and a zeroed block before it is asked for anything. What it is for is the same — a leak is a number that climbs while a frame repeats — and it falls to zero after dispose just the same. How many distinct sampler objects this device has made.

Diagnostic, and the number that says whether samplerFor is doing its job: a frame binding a hundred textures with SamplerOptions.linearRepeat should leave this at one. Read by webgpu_draw_test.dart, which is the only way to tell a cache hit from an allocation that happened to look the same.