feat: expose SmallOS adapters through SmallServer

[MikiEEE]

Summary

• export ThreadAdapter and AsyncioAdapter from the smallserver public API
• export the complete adapter error family from smallserver
• preserve lazy loading while keeping SmallOS as the execution backend
• migrate examples and integration tests away from SmallPackage adapter imports
• document the SmallServer-owned facade and add a public typing fixture

Usage

from smallserver import AdapterError, AdapterRegistry, ThreadAdapter

services = AdapterRegistry(
    blocking=ThreadAdapter(max_workers=2, max_pending=8),
)

Validation

• full suite: 182 tests passed
• no-extra suite: 182 tests passed with expected optional skips
• adapters demo completed with thread and persistent asyncio-loop results
• public star-import facade smoke passed
• compileall, documentation tests, and git diff --check passed