t23: add OpenIMP capture and isolated Helix encoding

[matteius]

Summary

• add the recovered T23 IMP ABI for FrameSource, ISP tuning, and encoder structures
• add T23 platform services and the 227/227 RVD IMP export surface
• isolate the proprietary T23 Helix /dev/soc_vpu encoder in an OEM-linked helper process
• exchange NV12 input and Annex-B H.264 output over tmpfs shared memory while OpenIMP owns capture and stream lifecycle
• add a reproducible T23 cross-build script and keep audio APIs out of OpenIMP so RAD can use the OEM audio stack independently

Device validation

Validated on cinnado_d1_t23n_sc2336_atbm6012bx with the recovered open-tx-isp T23 module:

• 1920x1080 H.264 High profile
• stable 15 fps under a concurrent LightNVR RTSP client
• 891 video packets over 59.933 s (14.85 fps), all video deltas 60-73 ms
• 2813 AAC packets over 59.989 s, all audio deltas 21-22 ms
• zero audio cadence outliers or non-monotonic timestamps
• full stream decode completed without codec errors

The helper deliberately loads /usr/lib/libimp.so; RVD loads the OpenIMP candidate. Separate RMEM arenas prevent the two allocators from colliding. T23 audio remains provided by the OEM libimp through RAD.