P2: implement vx_user_data_object data type (all 7 KHR functions) #104

Job	Run time
Build Khronos OpenVX sample	45s
build	2m 32s
Build rustVX (main)	2m 12s
data-objects	8s
graph	10s
vision-features	35s
image-ops	10s
vision-statistics	9s
user-data-object (KHR extension)	8s
vision-arithmetic	24s
vision-filters	1m 1s
vision-geometric	34s
enhanced-vision (Phase 1 — Min/Max)	9s
vision-pyramid	12s
Perf gate (PR vs main)	3m 9s
vision-color	11s
baseline	9s
Benchmark & compare (rustVX vs Khronos sample)	4m 59s
	17m 37s

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