v0.5.2

Patch release over 0.5.1. A correctness and robustness pass over the
VolumeRaycaster, plus two rsdm data-engine race fixes surfaced by the
cross-platform CI. adl2rsdm is re-released in lockstep.

Fixed — rsplot (VolumeRaycaster)

• Dark fringe around opaque regions gone (premultiplied-alpha compositing).
• Opacity no longer depends on the ray step count (Beer–Lambert normalized).
• set_steps clamped to [1, 4096] (GPU-timeout watchdog safety).
• Invalid volumes rejected up front (extent + RGBA-length validation).
• Ctrl/Cmd-pan tracks the pointer one-to-one (anchored at box-centre depth).
• Same VolumeId painted twice in one frame keeps each camera (per-callback uniforms).

Changed — rsplot

• VolumeRaycaster::remove / remove_volume_raycaster(id) free a volume's GPU resources.
• Dropped the unused cam_pos uniform.

Fixed — rsdm

• ca:// no longer leaks a duplicate initial sample when the connect handler re-enters (native-type-change on first connect); the connect-time value is gated on an actual change.
• calc:// always publishes its initial derived value regardless of child connect order (stamp changes folded in only once every child is connected).

Full notes: see CHANGELOG.md.