Fix diffusion flag leak, sd-cli orphan, and native family fallback

Six correctness fixes to the diffusion stack, found reviewing the merged
phase PRs on this branch:

- load_pipeline: restore the try/finally guard around the speed/quant/
  placement span. A failure after apply_speed_optims (e.g. OOM in quant or
  the memory plan) left TF32/cudnn flags flipped process-wide and the
  half-built pipe resident in VRAM. Now restores the flags and frees VRAM
  on a failed load.
- sd-cli Popen binds to the parent (PR_SET_PDEATHSIG via child_popen_kwargs,
  matching the llama.cpp sites), so a parent crash mid-generation can't
  orphan it holding VRAM/RAM.
- Native begin_load uses the filename-fallback family detector the route
  validated with, so a local .gguf whose family keyword lives only in the
  basename no longer dead-ends 400 on a no-GPU host.
- Generate error handler matches exact sentinel messages instead of the
  "cancelled" substring, fixing a 409 misroute and a raw sd-cli output leak.
- find_sd_cpp_binary honors UNSLOTH_STUDIO_HOME/STUDIO_HOME like the
  installer, so a custom Studio home resolves.
- Drop the redundant _tf32_prev bookkeeping; snapshot/restore_backend_flags
  is now the single owner of the TF32/cudnn restore.

The two client-state messages are now shared constants so the 409-vs-500
contract can't drift. Adds a regression test for each behavioral fix.

studio/backend/tests/test_sd_cpp_backend.py

@@ -219,6 +219,17 @@ def test_begin_load_requires_gguf_filename():
         b.begin_load("unsloth/Z-Image-Turbo-GGUF")
 
 
+def test_begin_load_resolves_family_from_filename_only(monkeypatch):
+    # A local .gguf pick whose family keyword lives only in the basename (parent dir
+    # carries none) must resolve via the same filename fallback the route validated
+    # with -- not dead-end with "Could not infer" on a native (no-GPU) host.
+    b = SdCppDiffusionBackend(engine = _FakeEngine())
+    monkeypatch.setattr(b, "_run_load", lambda **kwargs: None)  # skip the download thread
+    b.begin_load("/models/gguf-store", gguf_filename = "Z-Image-Turbo-Q4_K_M.gguf")
+    # Validation passed (no ValueError) and the family was inferred from the filename.
+    assert b._loading is not None and b._loading.repo_id == "/models/gguf-store"
+
+
 def test_ensure_binary_returns_found(monkeypatch):
     monkeypatch.setattr(bk, "find_sd_cpp_binary", lambda: "/usr/bin/sd-cli")
     assert ensure_sd_cpp_binary() == "/usr/bin/sd-cli"