[3.1-RC1] Add a serialization-format coverage matrix and define canonical ordering

[henrylyons]

Affected Sections: docs/serializations.md; serialization/jsonld/validation.md

Suggested Change:
Add a serialization-format coverage matrix (full/partial/not supported) for JSON-LD, RDF, Canonical, and legacy tag-value, and define the collation/algorithm behind the canonical 'ordered by name' rule. Add a table mapping each new class to its constraining mechanism (SHACL, JSON Schema, or prose-only). (docs/serializations.md; serialization/jsonld/validation.md)

Notes:
This feedback was provided by Microsoft at the SPDX working group meeting on June 30, 2026.