Reorganize: move studio services to editor folder, group by functionality

Goal

Reorganize the Studio codebase by moving many services from studio/services/ into the editor/ folder, making the studio project more focused on UI-only concerns. Additionally, reorganize the editor/ folder by grouping files by general functionality rather than having a flat structure.

Context / integration points

• studio/services/ contains many modules that are not inherently UI-related and could live in editor/.
• The editor/ folder has grown organically and lacks functional grouping.
• The goal is to make Studio a thinner UI shell, with editor logic in the editor/ crate/folder.
• This is an incremental refactoring — not everything needs to move at once, but we should identify candidates and establish the new structure.

Proposed structure

editor/
  project/          # Project management: open, close, settings, version migration
  assets/           # Asset import, catalog, meta, watcher
  scene/            # Scene editing, hierarchy, serialization
  ui/               # Editor panels, Inspector, Asset Browser, theme, i18n
  tasks/            # Background tasks, progress tracking
  input/            # Editor input handling, shortcuts
  dirty_check.zig   # Dirty asset tracking API (shared with window-close)
  ...

Tasks

• Audit studio/services/ and identify candidates for moving to editor/
• Propose the new editor/ folder structure with functional groupings
• Move identified modules (with deprecation shims or direct updates)
• Update all imports and build paths in Studio
• Verify everything still compiles and works
• Remove old files after confirming migration
• Document the new structure in a brief README or ADR

Acceptance criteria

• Studio contains primarily UI code (panels, docking, shell).
• Editor logic lives in editor/ with clear functional grouping.
• All existing functionality continues to work.
• The new structure is documented.