Build a Format Crate

Outline (to be expanded):

• Architecture: reader/writer, solver integration, error strategy (miette::Diagnostic).
• Using Shape/Peek to read; Partial to construct.
• Attribute plumbing: parse extension attributes, map to behavior.
• Testing patterns: golden files, round-trips, fuzzing, span assertions.
• Performance knobs: buffering vs streaming, zero-copy where possible.
• Walkthrough target: facet-json/facet-yaml excerpts.