Import the JBD Phase2 checkpoint for EXT4 journaling and concurrency validation.

Main changes:
- improve JBD2 transaction handling and journal coordination paths
- update EXT4 metadata operations for concurrent journaled workloads
- refine allocation guards, inode updates and extent-related paths
- add JBD Phase2 concurrency test program and runner
- update xfstests and benchmark scripts for the new validation stage
- refresh JBD Phase2 design, lock-order and milestone documentation
- remove local agent notes, repro notes and environment-only files from the submission
- ignore generated benchmark assets, logs and image artifacts

Validation:
- review staged files to avoid benchmark assets and raw logs
- keep generated cpio/images/log files out of the GitLab submission
- remove nonessential local workflow and repro-note files from the submitted tree
- include Phase2 concurrency tests and lock-order documentation

Contest stage:
- JBD Phase 2: concurrency corr...