fix: enable branch creation on tables that uses lance-namespace

[dantasse]

Creating a branch fails on tables that use lance-namespace. On a lance-namespace table, this happens in order:

1. write the manifest to a staging path under /tree//_versions/…manifest-
2. call the namespace's create_table_version to record that version, reading _refs/branches/.json to find the location of the branch
3. Write _refs/branches/.json

So step 2 fails because .json isn't written yet. (the commit handler mislabels this as "dataset already exists"). So this PR adds resolve_branch_location_unchecked, which just computes the main branch's path and adds tree/<branch> which is the right location.

(On unmanaged tables, step 2 doesn't happen, so there's no problem.)

Edit: dir::tests::test_branch_ops_reject_zombie_branch fails; this isn't ready yet