fix(adapter): re-export adapter classes from package entry points

[pasevin]

Summary

• Re-export EvmAdapter, StellarAdapter, and PolkadotAdapter classes from their respective package entry points (index.ts)
• These exports were accidentally removed during the ecosystemDefinition refactor in feat: add EVM adapter access control module #338, breaking exported app builds that import adapter classes directly
• The MidnightAdapter was unaffected as it still had export * from './adapter'

Root Cause

PR #338 refactored adapters to use the ecosystemDefinition pattern with a createAdapter factory for the builder app's lazy loading. During this refactor, the export { XAdapter } lines were replaced with import { XAdapter } (for internal use in createAdapter) but the re-export was not preserved. The exported app code generator produces import { EvmAdapter } from '...' which requires the class to be publicly exported.

Test plan

• All three adapter packages build successfully with tsup
• Adapter classes confirmed present in generated .d.ts type definitions
• Pre-commit hooks pass (lint, format, adapter compliance)
• CI passes
• Verify exported app builds for EVM, Stellar, and Polkadot ecosystems