feat(graphics): expose WGSL packed_4x8_integer_dot_product as a device cap

[mvaligursky]

Adds detection and automatic wiring for the WGSL packed_4x8_integer_dot_product language feature, which exposes the DP4a built-in functions for 8-bit packed integer dot products:

• dot4U8Packed, dot4I8Packed — packed 4×i8/u8 dot product
• pack4xI8, pack4xU8, pack4xI8Clamp, pack4xU8Clamp — pack helpers
• unpack4xI8, unpack4xU8 — unpack helpers

These accelerate quantized inference and similar integer-heavy compute workloads on hardware that exposes the DP4a instruction.

Follows the same pattern as unrestricted_pointer_parameters (#8785) and pointer_composite_access (#8786).

Changes:

• GraphicsDevice.supportsPacked4x8IntegerDotProduct flag, probed from navigator.gpu.wgslLanguageFeatures in WebgpuGraphicsDevice#initDeviceCaps.
• CAPS_PACKED_4X8_INTEGER_DOT_PRODUCT shader define for conditional compilation (#ifdef).
• requires packed_4x8_integer_dot_product; directive automatically injected into WGSL shaders on supporting devices via ShaderDefinitionUtils.getWGSLEnables.

Notes:

• Infrastructure-only — no shader callers yet. The cap is available for future compute shaders that benefit from DP4a (e.g. ML inference, image processing).
• On devices that don't expose the feature, the cap is false and no requires directive is emitted, so existing portable shaders continue to compile.