mesa-panvk-bifrost: iter13 — implement VK_EXT_transform_feedback for Bifrost

iter12 hit a wall: Brave's ANGLE-Vulkan path requires GLES3, which
requires VK_EXT_transform_feedback, which PanVk-Bifrost did not
implement. This iter implements that extension, unlocking the full
ANGLE-Vulkan-on-Bifrost stack.

The implementation follows Panfrost-Gallium's well-validated XFB lowering
(nir_io_add_intrinsic_xfb_info + pan_nir_lower_xfb) wired into the PanVk
shader pipeline after nir_lower_io. Adds 4 XFB buffer address sysvals
plus per-draw num_vertices to the graphics sysval struct. Buffer state
is tracked on the cmd buffer; per-draw sysval upload populates either
the bound buffer's GPU address or PAN_SHADER_OOB_ADDRESS (memory-sink)
so XFB-capable pipelines used outside Begin/End survive without GPU
fault — the Panfrost-Gallium idiom from gallium/drivers/panfrost/
pan_cmdstream.c:1350.

Verified on PineTab2 (Mali-G52 r1 MC1, RK3566):
- /tmp/panvk-iter13/probe_xfb: 3 vertices captured byte-exact
- /tmp/panvk-iter13/probe_xfb_nodraw: XFB pipeline used without Bind/
  Begin/End survives — DEVICE_LOST regression closed
- Brave 148 with --use-angle=vulkan: WebGL 2.0 (OpenGL ES 3.0) creates
  cleanly, renderer reports
  "ANGLE (ARM, Vulkan 1.2.335 (Mali-G52 r1 MC1), panvk)"
- chrome://gpu graphics feature status: Canvas/Compositing/OpenGL/
  Rasterization/WebGL/WebGL2/WebGPU/Video Decode all hardware accelerated

Phase docs:
- ~/src/panvk-bifrost/phase4_iter13_close.md  (build green)
- ~/src/panvk-bifrost/phase5_iter13_close.md  (review fixes applied)
- ~/src/panvk-bifrost/phase6_iter13_close.md  (Brave integration green)

pkgver bumped 26.0.6.r2 -> 26.0.6.r3; iter13 patch applied via
unified-diff (the 328-line change scope is past sed-of-individual-
lines territory). Sanity checks in prepare() verify the patch landed.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

arch/mesa-panvk-bifrost/PKGBUILD

@@ -30,7 +30,7 @@
 
 pkgname=mesa-panvk-bifrost
 _mesaver=26.0.6
-pkgver=26.0.6.r2
+pkgver=26.0.6.r3
 pkgrel=1
 pkgdesc="Patched Mesa libvulkan_panfrost.so exposing Bifrost-gen Mali to Vulkan apps (panvk-bifrost campaign)"
 arch=('aarch64')
@@ -79,6 +79,7 @@ source=(
     "https://archive.mesa3d.org/mesa-${_mesaver}.tar.xz"
     "0001-panvk-expose-robustness2-nullDescriptor-bifrost.patch"
     "0002-panvk-expose-vulkan-1.1-1.2-on-bifrost.patch"
+    "0003-panvk-bifrost-vk-ext-transform-feedback.patch"
     "brave-vulkan"
     "icd.json"
 )
@@ -88,6 +89,7 @@ sha256sums=(
     'SKIP'
     'SKIP'
     'SKIP'
+    'SKIP'
 )
 
 prepare() {
@@ -107,12 +109,23 @@ prepare() {
     sed -i 's|bool has_vk1_1 = PAN_ARCH >= 10;|bool has_vk1_1 = true;|' src/panfrost/vulkan/panvk_vX_physical_device.c
     sed -i 's|bool has_vk1_2 = PAN_ARCH >= 10;|bool has_vk1_2 = true;|' src/panfrost/vulkan/panvk_vX_physical_device.c
 
+    # iter13: VK_EXT_transform_feedback implementation for Bifrost (PAN_ARCH<9).
+    # Applied as a real unified-diff patch — the change is too large for sed.
+    # Phase-doc context: ~/src/panvk-bifrost/phase{4,5,6}_iter13_close.md.
+    # Unlocks ANGLE-Vulkan → GLES3 → WebGL2 / WebGPU on Brave (chrome://gpu
+    # reports "Hardware accelerated" across the board for the affected paths).
+    patch -p1 < "${srcdir}/0003-panvk-bifrost-vk-ext-transform-feedback.patch"
+
     # Sanity-check the patches landed.
     grep -q "KHR_robustness2 = true," src/panfrost/vulkan/panvk_vX_physical_device.c
     grep -q "EXT_robustness2 = true," src/panfrost/vulkan/panvk_vX_physical_device.c
     grep -q "nullDescriptor = true," src/panfrost/vulkan/panvk_vX_physical_device.c
     grep -q "has_vk1_1 = true;" src/panfrost/vulkan/panvk_vX_physical_device.c
     grep -q "has_vk1_2 = true;" src/panfrost/vulkan/panvk_vX_physical_device.c
+    # iter13 sanity:
+    grep -q "EXT_transform_feedback = PAN_ARCH < 9," src/panfrost/vulkan/panvk_vX_physical_device.c
+    grep -q "pan_nir_lower_xfb" src/panfrost/vulkan/panvk_vX_shader.c
+    test -f src/panfrost/vulkan/jm/panvk_vX_cmd_xfb.c
 }
 
 build() {