`Syntax.group_source_properties` does not round trip

Example:

block:
  let g :~ Group = 'foo bar baz'
  let new_g :~ Group:
    let (prefix, content, tail, suffix):
      g.group_source_properties()
    g.group_source_properties(prefix, content, tail, suffix)
  values(
    g.to_source_string(),
    new_g.to_source_string(),
  )

This gives "foo bar baz" vs. "". Compare with Syntax.source_properties:

block:
  let t :~ Term = 'foo'
  let new_t :~ Term:
    let (prefix, content, tail, suffix):
      t.source_properties()
    t.source_properties(prefix, content, tail, suffix)
  values(
    t.to_source_string(),
    new_t.to_source_string(),
  )

This gives "foo" vs. "foo" as expected.

Start by running the issue's Rhombus example and compare Syntax.group_source_properties with Syntax.source_properties. Trace the group_source_properties entry point to determine why reconstructing a Group loses its content, then verify that the original and reconstructed values produce matching source strings.