docs: define vNext language-service contracts

[Light2Dark]

Defines the accepted vNext capability charter and ADR 0001 before runtime implementation. The decision establishes a shared framework-independent language service, one disposable session per document, atomic revision-guarded updates, original-document UTF-16 ranges, explicit request outcomes, scoped catalog epochs, provider source forms, and a thin CodeMirror lifecycle adapter.\n\nThe contracts were derived from the repository research and current marimo integration, then iterated through three fresh-context design reviews and two blocker-resolution loops. Local lint, typecheck, test integrity, workflow validation, diff checks, and aggregate coverage pass.

---

Summary by cubic

Adds ADR 0001 and a capability charter defining the vNext SQL language-service contracts. Establishes a framework-agnostic service with per-document sessions, atomic revision-guarded updates, original-document UTF-16 ranges, unified request outcomes, and a thin CodeMirror adapter ahead of runtime work.

• New Features
  • Shared SqlLanguageService and per-document SqlDocumentSession with atomic text/context updates.
  • Opaque session revisions; all results carry a revision and require a currency check before apply.
  • Standard UTF-16 half-open original-document ranges and validated incremental changes.
  • Unified request outcomes (ready, unavailable, cancelled, failed) with clear cancellation and provider reports.
  • Narrow async provider boundary (no editor/DOM), bounded sync budget, worker-friendly guidance.
  • Catalog epochs with scoped invalidation; template masking/mapping preserving original offsets; thin CodeMirror adapter; walking-skeleton scope (relation completion first).

^{Written for commit 66853b3. Summary will update on new commits.}

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	Unknown% (🎯 91%)	0 / 0
🔵	Statements	Unknown% (🎯 91%)	0 / 0
🔵	Functions	Unknown% (🎯 90%)	0 / 0
🔵	Branches	Unknown% (🎯 85%)	0 / 0

File Coverage

No changed files found.

Generated in workflow #664 for commit 66853b3 by the Vitest Coverage Report Action