Switch Legion to master branch and update

[elliottslaughter]

This is preparation for the various Realm branches, getting Legion up to date to avoid future build complications.

• Switch Legion from flexflow branch to master
• Update Legion to latest (as of 2025-12-03)

My understanding is that the contents of Legion's flexflow branch are as follows:

• Sets cfg_task_streams to 1 by default
  • This can be configured by -ll:streams and no longer requires a branch
• Customizes the value of LEGION_MAX_RETURN_SIZE
  • No longer required because the FlexFlow backend for Legion is defunct anyway and the future is moving to Realm (for which this setting is irrelevant)

Beyond this, Legion's master branch has at least one critical feature for using Realm moving forward:

• Newer Realm versions dlopen the library libcuda.so instead of hard linking it, which makes test integration much easier.

Therefore I think Legion's master branch should be sufficient for all purposes moving forward and we should stop tracking a custom branch.

---

This change is