Rich diagnostic formatter with source context and highlighting.

This formatter produces compiler-style error output with:

• Severity and error code header
• Source location with unicode box-drawing frame
• Highlighted source context with underlines and branching labels
• Notes and help suggestions

Example Output

error[E0001]: Type mismatch
   ╭─[lib/my_app.ex:15:10]
   │
14 │   add = fn x :: integer, y :: integer ->
15 │     x + y + 1.5
   •             ─┬─
   •              ╰── expected `integer`, found `float`
   │
   ╰─────
      note: `+` with integer arguments returns integer
      help: consider using `trunc(1.5)`

Multi-file Diagnostics

Labels carrying their own :source render as continuation frames inside the same diagnostic: the report's own file opens the frame with ╭─[...], each additional file is introduced with ├─[file:line:col] and rendered against its own source, and a single ╰───── closes the frame. Groups whose source is missing from the provided sources render header-only.