v0.5.0: channel map ownership, deep copies, and schema-accurate validation

[jonnew]

• Flip the channel map direction from contact -> channel to channel -> contact. A hardware channel can only ever be wired to one contact, so channel is the value that actually needs uniqueness enforcement; keying by channel makes that structural (a Dictionary can't hold two entries for the same key), so duplicate-channel detection in Probe's constructor and ProbeGroup.ValidateDeviceChannelIndices no longer need a separate manual scan.
• Move channel map ownership off Probe: Probe.ChannelMap and Probe.TryGetMappedChannel are now internal. Public access goes through ProbeGroup.ChannelMap / ProbeGroup.TryGetMappedChannel, or through the new SingleProbeGroup base class for groups that always contain exactly one probe. Probe.GetChannelMap() and ProbeGroup.GetChannelMap() are removed in favor of the ChannelMap properties.
• Add SingleProbeGroup: an abstract ProbeGroup subclass that enforces the single-probe invariant and exposes Probe, ChannelMap, and TryGetMappedChannel without a probe-index argument.
• Add deep copy constructors to Probe, ProbeAnnotations, and ProbeGroup. Copies no longer share a channel map or contact/probe annotation store with their source.
• Add schema-accurate validation to Probe's JSON constructor:
  • ndim must be 2 or 3
  • contact_positions, contact_plane_axes, and probe_planar_contour rows must match ndim
  • contact_plane_axes entries can't be null and must contain exactly 2 axis vectors
  • contact_shape_params must be consistent with contact_shapes (circle requires radius, rect requires width and height, square requires width)
• Add validation to ContactShapeParam: radius/width/height must be non-negative, and at least one of radius or width must be specified.
• Require model_name and manufacturer in ProbeAnnotations, matching the probeinterface schema.
• Add ProbeGroup.NumberOfProbes.
• Enable InternalsVisibleTo for OpenEphys.ProbeInterface.NET.Tests.
• Correct and complete XML doc comments across the public API.