docs: define vNext language-service contracts (#171)

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.

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---
## 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).

<sup>Written for commit 66853b3.
Summary will update on new commits.</sup>

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Author: Light2Dark