feat: Add @af/sweph-core package for multi-platform architecture

## Phase 1: Core Extraction

- Create packages/core with shared types, interfaces, and utilities
- Add ISwephAdapter interface for platform-agnostic native bindings
- Add ISwephInstance interface for unified high-level API
- Extract pure JS utils: normalizeLongitude, getRashi, getNakshatra, etc.
- Copy constants: PLANETS, AYANAMSA, RASHIS, NAKSHATRAS
- Setup pnpm workspace configuration
- Core package builds successfully

This prepares for multi-platform support (Node.js, WASM, React Native)
with shared calculation logic and unified interfaces.

Author: bishalg

packages/core/dist/constants.d.ts

@@ -0,0 +1,10 @@
+/**
+ * @af/sweph-core
+ *
+ * Shared types, interfaces, and utilities for @af/sweph multi-platform library.
+ * This package has no native dependencies and can be used in any JavaScript environment.
+ */
+export * from './types';
+export * from './constants';
+export * from './utils';
+//# sourceMappingURL=index.d.ts.map