feat: add host-side integration test for Go DNS+dial overlay

Add integration_go_dns_dial.rs that builds the Go guest fixture with
TinyGo (WASI P1 reactor), registers a manual warpgrid_shim.dns_resolve
host function with a test service registry, and exercises 5 test cases:

- Service registry hostname resolution (AC1)
- Multiple A record resolution in order (AC4)
- Nonexistent hostname error handling
- IP literal bypass
- DNS error wrapping as *net.OpError{*DNSError} (AC3)

All 5 tests pass end-to-end through the full shim ABI:
Go → shim_wasi.go → //go:wasmimport → host function → guest memory.

Refs #42

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: nadilas