docs: add stats dashboard design docs, plans, and knowledge base

- Stats dashboard redesign design and implementation plans
- Episode detail and Anki card link design
- Internal knowledge base restructure
- Backlog tasks for testing, verification, and occurrence tracking

docs/architecture/layering.md

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+<!-- read_when: adding dependencies, moving files, or reviewing architecture drift -->
+
+# Layering Rules
+
+Status: active  
+Last verified: 2026-03-13  
+Owner: Kyle Yasuda  
+Read when: deciding whether a dependency direction is acceptable
+
+## Preferred Dependency Flow
+
+1. `src/main.ts`
+2. `src/main/` composition and runtime adapters
+3. `src/core/services/` focused services
+4. `src/core/utils/` and other pure helpers
+
+Renderer, launcher, plugin, and stats each keep their own local layering and should not become a grab bag for unrelated cross-runtime behavior.
+
+## Rules
+
+- Keep `src/main.ts` thin; wire, do not implement.
+- Prefer injecting dependencies from `src/main/` instead of reaching outward from core services.
+- Keep side effects explicit and close to composition boundaries.
+- Put reusable business logic in focused services, not in top-level lifecycle files.
+- Keep renderer concerns in `src/renderer/`; avoid leaking DOM behavior into main-process code.
+- Treat `launcher/*.ts` as source of truth for the launcher. Never hand-edit `dist/launcher/subminer`.
+
+## Smells
+
+- `main.ts` grows because logic was not extracted
+- service reaches directly into unrelated runtime state
+- renderer code depends on main-process internals
+- docs-site page becomes the only place internal architecture is explained