refactor: move distributor core into fastforge

[lijy91]

Summary

• move the distributor implementation from unified_distributor into fastforge
• remove the unified_distributor package from the workspace
• make flutter_distributor depend directly on fastforge
• bump fastforge to 0.6.12 and flutter_distributor to 0.6.11
• update package changelogs and migration documentation

Validation

• dart analyze for fastforge and flutter_distributor
• dart test for fastforge
• dependency_validator for fastforge
• CLI smoke tests for fastforge and flutter_distributor
• pub publish dry-run with zero warnings
• real macOS Flutter build and ZIP packaging