Generate API reference for invisible-wallet-sdk

## Summary
The SDK has no public API docs. Generate them from TypeScript source using TypeDoc.

## Files
- `sdk/`

## Acceptance criteria
- [ ] Add `typedoc` as a devDependency
- [ ] Add `docs` script in `sdk/package.json`
- [ ] Generate to `sdk/docs/` (gitignored) — or commit to `docs/sdk/`
- [ ] Add a CI step (optional) that publishes to gh-pages