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

Start with the shared language-server MVP in #393 and the tracked work in #399, then inspect the generated xw.v1 editor metadata and the Relay registration catalogue. Validate the implementation against a golden project workspace and the listed acceptance criteria, including safe rename validation, Rhai and CEL assistance, parser regressions, and equivalent VS Code and Zed behavior.

Written by the indexing model from the issue text.