Fix v4l2loopback exclusive_caps, YU12->NV21 conversion, cameraserver crash

- Add exclusive_caps=0 to v4l2loopback insmod params to allow multiple
  simultaneous readers (HAL + verification capture)
- Fix YU12->NV21 pixel format conversion in V4L2 reader thread
- Fix YUYV->NV21 conversion as well
- Fix cameraserver crash by setting inputBuffer.streamId = -1
- Enable Hal Buffer Manager for requestStreamBuffers support
- Add JPEG_MAX_SIZE and JPEG_AVAILABLE_THUMBNAIL_SIZES metadata
- Add availableRequestKeys and availableResultKeys metadata
- Fix consumerUsage=0 for output streams (Camera3Device requirement)
- Fix metadata buffer overflow (2048->4096 bytes)

webroot/style.css

@@ -0,0 +1,28 @@
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+.container{display:flex;flex-direction:column;gap:12px}
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+.src-btn.active{background:#1f6feb33;border-color:#1f6feb;color:#58a6ff}
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+.row{display:flex;gap:8px;margin-bottom:12px}.row>div{flex:1}
+.num{width:100%}
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+.grid{display:grid;grid-template-columns:1fr 1fr;gap:8px}
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