Project slugs: stable project identifiers in editor URLs (replaces the sandbox in-memory project-id map)

As a developer moving around a workflow's versions and sandboxes, I want projects (including sandboxes) to have stable slugs used in editor URLs, so that navigation and cross-project diffing don't depend on the project UID.

What's broken

The collaborative editor URL carries the project UID. A sandbox is a separate project with a different UID, so moving between a live workflow and its sandbox means switching projects and URLs. There is no stable, human-meaningful identifier that a sandbox can share with its parent.

Sandbox DevX Phase 1 (in-editor version and sandbox switching, #4865) ships an interim in-memory sandbox-to-project map as a deliberate shortcut. This is the proper fix that replaces it.

What to do

Introduce a slug on projects and use it in editor routes, so a sandbox can carry its parent's slug and switching does not need the in-memory map. Keep UID routes working during the transition, and backfill slugs for existing projects.

Why this is standalone, not part of the Sandbox DevX epic

Slugs are app-wide routing infrastructure, broader than sandboxes, and they also unlock proper cross-project and cross-version diffing (currently blocked on comparing across different UIDs). Tracking it on its own so it is reused platform-wide rather than scoped to one epic.

Acceptance criteria

• Editor URLs use project slugs.
• Switching between a workflow's sandboxes and versions works via slugs, without the interim in-memory map.
• Existing projects get slugs (backfill); old UID URLs keep working during the transition.
• The interim map in #4865 can then be removed.

Links

Replaces the interim map in #4865. The map-versus-slugs direction is one of the open decisions tracked in #4922. Enables the cross-UID diff work.

Start by reading #4865 and the map-versus-slugs decision tracked in #4922; the issue names no files or tests. Trace the editor URL and project/sandbox routing entry points, then verify slug-based switching, UID URL compatibility, existing-project backfill, and removal of the interim map.