Add ShapeIndex encoding and decoding

Normalized profile

What this task asks for

Add stable ShapeIndex Encode/Decode support so indices round-trip without rebuilding. Add `Encode` to `io.Writer` and `Decode` from `io.Reader` on `ShapeIndex`. All built-in `Shape` types must round-trip. Shape IDs must survive encoding so cell references stay valid. The full spatial cell structure must be preserved so queries and iteration work without `Build`. A ShapeIndex encoded without an explicit `Build` must still decode completely. Decoding malformed input must return errors rather than panicking, including truncated data, corrupted bytes, and oversized allocation requests.