[jordanella]

When the Linux renderer fails to load, the log says only (2). That covers a missing file and a hookshot_vk.so from another build alike, though the second is the one that needs explaining: it takes symbols from the executable, so it fails at dlopen, before mkwvk_init's version check, and the game quietly falls back to WebGPU.

Change

• host/posix/win32_posix.c: LoadLibraryA keeps dlerror()'s text for the last failure, per thread, and hos_load_error() returns it ("" after a success). GetLastError() is still ERROR_FILE_NOT_FOUND, as before.

• host/rx_backend.c: load_vulkan's failure line prints that text on Linux:

  rx: RX_BACKEND=vulkan, but .../hookshot_vk.so did not load (.../hookshot_vk.so: undefined symbol: mkwvk_...): WebGPU
  

  Windows keeps the error number, which already tells a missing file (126) from a missing function (127).

Part of the dev-loop proposal's B1 (#2636): a renderer swap during development needs this line to say why the new one was refused.

Testing

• tests/test_renderer_load_error.py (new):

  • everywhere: the source keeps the text and logs it (it fails on main);
  • on Linux: builds win32_posix.c as it is and loads, through LoadLibraryA, a .dll name with no library behind it, a library that needs a symbol nothing has, then a good one.

• The Linux part, run by hand on WSL Ubuntu (gcc), as the check does it:

  failed|2|<tmp>/absent_vk.so: cannot open shared object file: No such file or directory
  failed|2|<tmp>/needs.so: undefined symbol: mkwvk_from_another_build
  loaded|0|
  

• Every check that reads the changed files (37), through run_all.py, rebased on today's main, on Windows: 34 pass; nx2/test_bc_storage_views.py, nx2/test_dll.py and nx2/test_leaf.py fail the same way on main here. The new check's Linux part is skipped on Windows.

• Not run: the game on Linux. The change is the failure path's log line.

See #2636 (B1).

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