fix(v2): enforce absolute protocol paths

[benbrandt]

Summary

• brand generated experimental-v2 AbsolutePath values so raw strings cannot enter typed outbound calls or handler responses
• add a host-independent constructor and type guard for POSIX, Windows drive, and UNC paths
• validate inbound absolute paths in generated Zod schemas
• keep the generator authoritative for both the branded type and runtime validator

This changes only the existing experimental v2 surface. It should land after the generated-source gate PR because both update the generator.

Testing

• npm run generate -- --skip-download
• npm run check (870 tests)