Honor custom catalog paths when activating translations

[muneikh]

Activating the same locale from a second catalog directory currently keeps using the first directory’s translation. Load explicit catalog paths through gettext’s file cache and store the active translation in thread-local state, while retaining the existing default-catalog cache.

The regression uses two real GNU catalogs and checks switching between both directories through activate() and apnumber(). It fails before the fix. With locale catalogs compiled, the full suite passes (857 tests); Ruff, Black and focused Mypy checks pass.

[Wkhanex]

Two real temporary gettext catalogs for the same locale now remain distinct by path. Switching A→B→A selects the expected translation, a worker thread can activate B without changing the main thread’s A, and deactivation restores the untranslated text. Explicit custom paths do not enter the default-path cache.

Checked ecc61d994fa6 with 7 standalone assertions on Python 3.12.14. I haven't run the full suite. The generated _version module was supplied as metadata only.