Sync uv.lock to the 0.3.9 release on the maintenance line

[alexeygrigorev]

The 0.3.9 release commit (78f2054) bumped pyproject.toml to 0.3.9 but left uv.lock pinning the editable community-base package at 0.3.7. A fresh uv run on the maintenance line regenerates the lock, which is exactly the drift sitting uncommitted in the c62a-maint scratch worktree.

This pull request distills that repair into one focused change: the community-base entry in uv.lock moves to 0.3.9. The released exclude-newer freeze timestamp is kept as is, and no other lock entry changes.

Verification:

• git diff: one line changed, uv.lock 2 +-, community-base version 0.3.7 to 0.3.9 (nh3 at 0.3.7 untouched)
• uv lock --check on the branch: Resolved 54 packages, exit 0, lock not rewritten
• python scripts/plan.py check: OK, 109 issues, STATUS.md consistent

No package code changes, so the site suites are unaffected; the pull request CI runs the cross-repo check against both sites' default branches as usual.