Add safe rename, references, and embedded Rhai/CEL editor assistance

Tracked by #399. Depends on #393.

## Outcome

The shared Registry Stack language server moves beyond MVP navigation to safe project-wide refactoring and useful assistance inside authored Rhai and CEL expressions.

Depends on the shared language-server MVP.

## Scope

- Find all references for integrations, entities, services, consultation aliases, outputs, claims, credential profiles, and fixtures.
- Prepare and apply safe project-wide rename edits.
- Reject renames that collide, violate identifier constraints, or would produce an invalid project graph.
- Use the generated `xw.v1` editor metadata for Rhai helper completion, hover, and signature help.
- Provide context-aware completion for project symbols referenced inside supported CEL and authored expression strings.
- Add targeted quick fixes where the compiler can identify an unambiguous correction.
- Keep expression assistance aligned with the released compiler/runtime ABI rather than a handwritten editor-only function list.

## Acceptance criteria

- [ ] Find references returns all and only semantic references across a golden project workspace.
- [ ] Rename produces a project that passes schema and compiler validation.
- [ ] Invalid, ambiguous, or security-sensitive renames are refused with an actionable explanation.
- [ ] Rhai `xw.v1` completion, hover, and signature help are generated from the same registration catalogue used by Relay.
- [ ] CEL completion is limited to symbols actually available in the selected authored context.
- [ ] Expression parsing never executes authored code or resolves credentials.
- [ ] Regression tests cover flow-style and block-style YAML, quoted scalars, aliases supported by the authoring parser, and multiple project roots.
- [ ] VS Code and Zed expose equivalent LSP-backed behavior where each editor supports the protocol capability.

## Non-goals

- A general-purpose Rhai or CEL IDE
- Runtime debugging
- Live source inspection

Beginne mit dem gemeinsamen language-server-MVP in #393 und der nachverfolgten Arbeit in #399, untersuche anschließend die generierten xw.v1-Editor-Metadaten und den Relay-Registrierungskatalog. Validiere die Implementierung anhand eines Golden-Project-Workspace und der aufgeführten Akzeptanzkriterien, einschließlich der sicheren Umbenennungsvalidierung, der Unterstützung für Rhai und CEL, der Parser-Regressionen sowie eines gleichwertigen Verhaltens in VS Code und Zed.

Vom Indexierungsmodell aus dem Issue-Text verfasst.