Update Rerun to 0.31.3

[KevinGliewe]

Bumps the vendored extern/rerun submodule from a tip-of-main snapshot to
the stable 0.31.3 release tag (commit 42eeab2e), and bumps the package
version to 0.31.3.

The 0.31.x release line never included the cors_allow_origins parameters
that were added on main, so the rr_recording_stream_serve_grpc P/Invoke
binding and its callsite drop those two trailing arguments. The public
ServeGrpc(...) C# API already did not expose CORS, so this is not a
public-API break.

Also fixes three pre-existing codegen bugs that were exposed by the new
blueprint types in 0.31.3:

• FbsParser now qualifies same-namespace shorthand references with the
  current namespace (e.g. "kind: TextLogColumnKind" inside a blueprint
  fbs is now stored as "rerun.blueprint.datatypes.TextLogColumnKind"
  rather than just "TextLogColumnKind"). This lets the registry resolve
  the reference and unblocks the next two fixes.

• DatatypeEmitter now handles enum-typed reference fields in multi-field
  tables (e.g. TextLogColumn { visible: bool, kind: TextLogColumnKind }):
  it casts to the underlying primitive and uses a primitive Arrow
  builder instead of calling a non-existent .ToArrow() on the enum.

• Program.cs now post-processes blueprint files so that "Datatypes.X"
  and "Components.X" references are rewritten to fully-qualified
  global::Rerun.Net.{...} or global::Rerun.Net.Blueprint.{...} paths.
  Without this, the closer Rerun.Net.Blueprint.{Datatypes,Components}
  sibling namespaces shadowed the core namespaces (e.g. blueprint
  archetype LineGrid3D referencing core Components.StrokeWidth would
  resolve to non-existent Rerun.Net.Blueprint.Components.StrokeWidth).
  ComponentEmitter.QualifyDatatypeRef was tightened to always emit the
  Datatypes. prefix so the post-processor has a consistent thing to
  rewrite; the resulting core component diff is purely cosmetic.

Net schema delta of regenerated .g.cs files: +32 brand-new blueprint
types (2 datatypes, 10 components, 20 archetypes), zero deletions.