feat: nonblocking buffer readback

[sebcrozet]

Added

• Non-blocking GPU→CPU readback. A new GpuReadback type stages a buffer copy and returns immediately: request/request_copy start the transfer and try_take polls for completion (is_idle reports whether a readback is in flight). GpuTimestamps gained matching request_read/try_take/is_idle for non-blocking timestamp readback. Both are driven by a new Backend::poll maintenance method (a non-blocking counterpart to synchronize) implemented across the WebGPU, CUDA, Metal, and CPU backends. The metal feature now pulls in block for command-buffer completion handlers.
• WebGpu::from_device: build a WebGPU backend on top of an already-created wgpu instance/adapter/device/queue instead of creating its own.
• from_wgpu constructors on GpuBufferSlice, GpuBufferSliceMut, and WebGpuBufferSlice to wrap a foreign wgpu::Buffer (owned by another library sharing the device) as a khal buffer slice.
• force_cpu_coroutines option for #[spirv_bindgen]: forces the CPU backend to dispatch a kernel through the coroutine scheduler so barrier_wait() synchronizes, even for kernels with no #[spirv(workgroup)] parameter. Replaces the old dummy-workgroup-parameter hack.

Fixed

• WebGPU entry-point lookup on wasm now mirrors naga's identifier sanitization, which appends a trailing _ to names ending in a digit, so kernels like reduce_add_f32 resolve correctly.