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* docs: add guide for AHB zero-copy input on Android Address #5531 - the AHB import API (VkAndroidHardwareBufferImageAllocator, ImportAndroidHardwareBufferPipeline) has no user-facing docs today and existing users hit linker errors against the official prebuilt because build-android.cmd targets -DANDROID_PLATFORM=android-21 which strips the AHB symbols. The new page covers build prerequisites (__ANDROID_API__ >= 26), AImageReader_newWithUsage setup, the per-frame call sequence, per-AHB- pointer pipeline caching (~5000x setup-cost reduction), and trouble- shooting for #5531 / #5190. * docs(ahb): add ex.input(VkMat) format-matching caveat + soften cache bound Two findings from a follow-up spike on Adreno 830: - ex.input(VkMat) does NOT auto-convert format. The previous draft's hint "// ex.input(\"data\", dst, cmd);" actually crashes if the network was loaded with use_fp16_storage / use_fp16_packed (common on Vulkan path) - NetPrivate::forward_layer recurses on itself looking for an implicit converter that doesn't exist, observed as a ~180-frame stack overflow at net.cpp:251. Add a dedicated caveat explaining the failure mode and the vkdev->convert_packing workaround, and weaken the snippet hint accordingly. - AImageReader's AHB pool isn't strictly bounded by maxImages. Under steady state it stays at maxImages (3 -> 3 cache_n in 36 consecutive cycles), but surface/focus events can transiently push it higher (observed cache_n=7 with maxImages=3 in one run). Soften the "<= maxImages entries" claim. * docs(ahb): note cross-vendor validation (Adreno + Mali) Same import path verified end-to-end on: - Adreno 830, Vulkan 1.3.284 (Galaxy S25 Ultra) - Mali-G925 Immortalis MC12, Vulkan 1.3.278 (Galaxy Tab S11) Both report VK_ANDROID_external_memory_android_hardware_buffer spec version 5 and surface AHARDWAREBUFFER_FORMAT_Y8Cb8Cr8_420 AHBs from the camera HAL. Only auxiliary usage bits differ between vendors and are not consumed by this path (Adreno: 0x20103, Mali: 0x20133).