Studio: fix dataset upload data loss, caption over-count, and custom-root sd.cpp uninstall

- Diffusion dataset upload now streams each file into a sibling temp file and
  atomically os.replace()s it into place only after the whole file is written and
  within the size cap. A mid-batch 413 (or any abort) removes the temp, never an
  example already stored under the same name, so re-uploading a too-large batch can
  no longer truncate or delete a previously uploaded image.
- _diffusion_dataset_summary counts an image as captioned only when it resolves to a
  non-empty caption via the same sidecar-over-metadata precedence the trainer uses. An
  empty (tombstone) sidecar shadows a metadata row and makes the trainer skip the
  image, so counting it over-reported caption_count and mislabeled an effectively
  uncaptioned dataset as captioned.
- uninstall.sh/.ps1 now remove a custom/env-mode Studio's native diffusion build that
  installs beside the root as a stable-diffusion.cpp sibling (find_sd_cpp_binary
  resolves it from the Studio home's parent), guarded by the same unsafe-path check,
  and stop processes locking the default-mode stable-diffusion.cpp before removing it.

Adds regression tests for the upload data-loss and caption-count paths and a hermetic
shell test for the custom-root stable-diffusion.cpp removal.

scripts/uninstall.ps1

@@ -366,7 +366,7 @@ function Uninstall-UnslothStudio {
     _StopStudioProcesses -KnownRoots $knownRoots
     # Also stop anything holding a handle on the exact paths we delete (llama-server,
     # the CLI shim, an mp-fork python with a venv DLL) so the dir delete isn't refused.
-    _StopProcessesLockingRoots -Roots (@($knownRoots) + @($defaultDataDir, $defaultLlamaCpp, $defaultCache, $defaultNode))
+    _StopProcessesLockingRoots -Roots (@($knownRoots) + @($defaultDataDir, $defaultLlamaCpp, $defaultSdCpp, $defaultCache, $defaultNode))
 
     # ── Remove custom-root install trees ──
     _Step "Removing data and install directories..."
@@ -380,6 +380,16 @@ function Uninstall-UnslothStudio {
             continue
         }
         _RemovePath $r
+        # Native diffusion (stable-diffusion.cpp) for a custom/env-mode Studio installs beside
+        # the root at <parent>\stable-diffusion.cpp -- find_sd_cpp_binary resolves it from
+        # UNSLOTH_STUDIO_HOME.parent (sd_cpp_engine.py) -- so removing only the root leaves the
+        # build behind. Derive and remove the sibling, guarding the parent path the same way.
+        $customSdCpp = Join-Path (Split-Path -LiteralPath $r -Parent) "stable-diffusion.cpp"
+        if (_IsUnsafeRoot $customSdCpp) {
+            _Substep "refusing to remove unsafe path: $customSdCpp" "Yellow"
+        } else {
+            _RemovePath $customSdCpp
+        }
     }
     # Default install dir (always at %USERPROFILE%\.unsloth\studio when present).
     if ($defaultStudioHome) { _RemovePath $defaultStudioHome }

scripts/uninstall.sh

@@ -210,6 +210,16 @@ _custom_studio_roots | while IFS= read -r _custom_root; do
         continue
     fi
     _remove_path "$_custom_root"
+    # Native diffusion (stable-diffusion.cpp) for a custom/env-mode Studio installs beside
+    # the root at <parent>/stable-diffusion.cpp -- find_sd_cpp_binary resolves it from
+    # UNSLOTH_STUDIO_HOME.parent (sd_cpp_engine.py) -- so removing only the root leaves the
+    # build behind. Derive and remove the sibling, guarding the parent path the same way.
+    _custom_sd_cpp="$(dirname "$_custom_root")/stable-diffusion.cpp"
+    if _is_unsafe_root "$_custom_sd_cpp"; then
+        echo "  refusing to remove unsafe path: $_custom_sd_cpp" >&2
+    else
+        _remove_path "$_custom_sd_cpp"
+    fi
 done
 _remove_path "$HOME/.unsloth/studio"
 # Default-mode shared llama.cpp build + cache are siblings of studio (not removed