chore: add Apache 2.0 copyright header to all source files

[tengqm]

Add standard Apache 2.0 copyright header (Copyright 2026 FlagOS Contributors) to all source files.

• ~3,100 files stamped with Apache 2.0 headers
• 473 files with existing upstream copyright (TensorFlow, HuggingFace, etc.) — left untouched
• 98 files with other licenses (MPL: 30, proprietary: 39, MIT: 13, BSD: 16) — third-party/upstream code
• 1 file with unlicensed copyright notice — flagged for review
• 508 files skipped by rule — binary, PDF, symlinks, etc.

This PR was written in part with the assistance of generative AI.