Editions: gate extension dtypes and support crate-declared families (json) #548

Rust editions gate extension dtypes as well as encodings, and edition families are no longer a fixed set: optional crates declare their own. vortex-java models neither.

Rust behavior

• An edition lists its members by component kind, and an extension dtype is one of those kinds (EditionMember::dtype(..)). A writer with an edition policy may emit only members of enabled families, so a schema with an extension dtype outside them is refused.
• Families are declared per crate. vortex-json/src/editions.rs declares the json family (origin vortex-json) with one draft edition, json2026.08.0, adding the vortex.json dtype. vortex_json::initialize(session) registers the dtype and enables the family together. Being outside core reflects that a reader built without vortex-json cannot resolve vortex.json.

vortex-java today

1. The edition guard (WriteOptions#editions(), ADR 0023) checks array encodings only, so withEdition(..) lets a schema carry vortex.json or any custom extension id (now easy to register, Pluggable extension types: register custom logical types on the read/write registries #530/Pluggable extension types on the read and write registries (#530) #547). docs/compatibility.md already notes the catalog models encodings only.
2. EditionFamily is a closed enum (CORE/PREVIEW/ZSTD), and CLAUDE.md justifies this because a private family carries no real guarantee. Rust now has crate-declared families (json), so the enum is missing one, and the closed-world rationale no longer matches the reference.

To decide

• Extend the Editions catalog with extension-dtype members and enforce them in the writer's edition guard at VortexWriter.create.
• Model json (and decide between keeping EditionFamily closed with known Rust families added, and opening it the way Rust does). Update ADR 0023 / CLAUDE.md accordingly.
• Related: a built-in vortex.json ExtensionDecoder<String> (UTF-8 storage, validate rejecting non-Utf8), so JSON files open without allowUnknown() after Pluggable extension types on the read and write registries (#530) #547.

Refs #530