docs(04): create phase plan

Phase 04: Theme Engine
- 2 plans in 2 waves
- Wave 1: Theme loading and layout composition (04-01)
- Wave 2: Asset pipeline and Vue integration (04-02)
- Ready for execution

.planning/ROADMAP.md

@@ -84,11 +84,11 @@ Plans:
   3. Asset management bundles and serves CSS/JS from theme
   4. Theme can declare rendering mode (SPA, hybrid, static)
   5. Vue components integrate into theme pages
-**Plans**: TBD
+**Plans**: 2 plans
 
 Plans:
-- [ ] 04-01: Theme loading and layout composition
-- [ ] 04-02: Asset pipeline and Vue integration
+- [ ] 04-01-PLAN.md - Theme loading and layout composition (ThemeConfig, ThemeLoader, ThemeService, custom Pebble tags)
+- [ ] 04-02-PLAN.md - Asset pipeline and Vue integration (AssetService, ViteManifest, StaticPageCache, PagePipeline)
 
 ### Phase 5: CLI Scaffolding
 **Goal**: Provide developer tools for rapid project setup
@@ -209,7 +209,7 @@ Phases execute in numeric order: 1 -> 2 -> 3 -> 4 -> 5 -> 6 -> 7 -> 8 -> 9 -> 10
 | 1. Foundation | 3/3 | Complete | 2026-02-05 |
 | 2. Plugin System | 0/3 | Planned | - |
 | 3. Component System | 0/2 | Planned | - |
-| 4. Theme Engine | 0/2 | Not started | - |
+| 4. Theme Engine | 0/2 | Planned | - |
 | 5. CLI Scaffolding | 0/2 | Not started | - |
 | 6. Backend Authentication | 0/3 | Not started | - |
 | 7. Admin Forms & Lists | 0/3 | Not started | - |