# 04 — Module loader & registry

## Goal

Implement the module system that everything else hangs off: each module declares a manifest, the loader composes them at startup, and core services iterate the registry instead of hardcoding entity types.

## Why

This is the load-bearing extensibility piece. Get this wrong and every later module needs core changes.

## Depends on

- 03 (DB)

## Scope

### Module manifest type (`src/modules/_core/module.ts`)

```ts
export type ModuleManifest = {
  id: string; // "calendar", "lists", "notes"
  name: string; // human-readable
  nav?: { href: string; label: string; icon?: string };
  entities: EntityTypeRegistration[]; // see below
  dashboardWidgets?: DashboardWidget[];
  quickAdds?: QuickAddAction[];
};

export type EntityTypeRegistration = {
  type: string; // "calendar.event", "lists.item", "notes.note"
  label: { singular: string; plural: string };
  share?: ShareCapabilities; // null = not shareable
  reminder?: ReminderCapabilities; // null = not remindable
  search?: SearchAdapter; // null = not searchable
  resolveUrl: (id: string) => string; // canonical app URL
  loadForShare?: (id: string) => Promise<unknown>; // payload for /s/<token>
};
```

### Registry (`src/modules/_core/registry.ts`)

- `registerModule(manifest)` and `getRegistry()` returning frozen views.
- `getEntityType(type)` lookup.
- Import-time side-effect free: modules are listed and registered explicitly in `src/modules/index.ts`.

### Loader (`src/modules/index.ts`)

- Imports each module's manifest and calls `registerModule`. Order is deterministic.
- Stub modules for now: `calendar`, `lists`, `notes` each export an empty-but-valid manifest so the loader has something to register.

### Wiring

- Root layout reads the registry to render nav.
- A throwaway `/debug/registry` page (dev-only) dumps the loaded registry as JSON.

## Out of scope

- Real implementations of share / reminder / search adapters (later phases).
- Dynamic plugin loading from disk. Modules are statically imported.

## Acceptance criteria

- [ ] `src/modules/index.ts` registers three stub modules.
- [ ] Nav renders entries from the registry, not from a hardcoded list.
- [ ] `/debug/registry` shows all three module manifests in dev.
- [ ] Adding a fourth stub module only requires creating its folder + adding one line to `src/modules/index.ts`.
- [ ] All types exported from `_core` so other modules can import without circulars.

## Notes

- Frozen objects (`Object.freeze`) on registry exposure are cheap insurance against accidental mutation.
- Resist the urge to make this a fancy DI container. A plain map is enough.