feat(vortex-edition): edition definitions crate with generated registry

[joseph-isaacs]

Rationale for this change

Implements the editions model introduced in #8852 as a single source of truth in code, so the published registry, writer enforcement, and freeze guarantees all derive from one place instead of being hand-maintained. Stacked on #8852.

What changes are included in this PR?

• vortex-edition crate (dependency-free except serde): the stored model is minimal — Edition { id, draft } (freeze date inferred from the identifier: core2026.07.0 → 2026-07) and the membership edge EncodingDecl { id, since, required_vortex_release }. Everything else is derived: status/lineage per family, the encoding set per edition, the delta vs. the superseded edition (members whose since equals the edition's id), and the edition-level required Vortex release (max over member edges when all are recorded, falling back to "first release containing the frozen edition").
• Validation: duplicate encoding/edition IDs, since references to unknown editions, family chronology (drafts newest), and malformed release strings all fail computation loudly.
• Freeze tests: FROZEN_CORE_2026_07_0 pins the exact 32-encoding set of the published edition; 8 unit tests total (freeze, invariants, derivation, JSON round-trip).
• cargo xtask generate-editions (alongside generate-fbs/generate-proto): writes one JSON definition and one Markdown page per edition — pages render from the re-parsed JSON so the two cannot disagree — and rewrites the registry index (table + hidden toctree) in docs/specs/editions.md between editions:index markers. The hand-written registry from docs: introduce Vortex editions for users of the file format #8852 becomes generated output.
• docs/developer-guide/internals/editions.md: contributor-facing notes on the data model, the required-release inference rules, the generation pipeline, and the stabilize-an-encoding / cut-an-edition workflows, linked from the spec page.

Checks run: cargo test -p vortex-edition (8 passed), cargo clippy -p vortex-edition -p xtask --all-targets, cargo +nightly fmt, git diff --check, and a clean Sphinx docs build.

Not yet wired up (documented in the internals page): deriving the writer's ALLOWED_ENCODINGS from Edition::current() with a with_edition API, the session cross-check test, release tooling that records the inferred required release, reader error messages linking the registry, and layout declarations.

What APIs are changed? Are there any user-facing changes?

New (unpublished-consumer) crate vortex-edition exposing EditionId, Edition, EncodingDecl, the EDITIONS/ENCODINGS statics, computed EditionManifests, and artifact generation. New cargo xtask generate-editions subcommand. User-facing docs gain generated per-edition pages with machine-readable JSON definitions. No existing APIs change.

🤖 Generated with Claude Code

https://claude.ai/code/session_01KdR42Svu74NcNzC7XJYwLr

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