fix: include full build stderr and CUDA diagnostics in error

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -245,8 +245,12 @@ def ensure_correlation_extension_installed():
     import torch
     cuda_home = os.environ.get("CUDA_HOME", "")
     nvcc_ok = shutil.which("nvcc") is not None
-    print(f"[DEBUG] torch.cuda.is_available={torch.cuda.is_available()}, "
-          f"CUDA_HOME={cuda_home!r}, nvcc_in_PATH={nvcc_ok}")
+    diag = (f"torch={torch.__version__}, cuda={torch.version.cuda}, "
+            f"torch.cuda.avail={torch.cuda.is_available()}, "
+            f"CUDA_HOME={cuda_home!r}, nvcc={nvcc_ok}")
+    print(f"[DEBUG] {diag}")
+    # Store diagnostics for error reporting
+    _diag_info = diag
 
     # Ensure CUDA_HOME is set so setuptools can find nvcc
     if not cuda_home:
@@ -282,11 +286,11 @@ def ensure_correlation_extension_installed():
             env={**os.environ},
         )
         if result2.returncode != 0:
-            print(f"[ERROR] PyPI install also failed: {result2.stderr[-500:]}")
             raise RuntimeError(
                 f"Cannot install spatial-correlation-sampler.\n"
-                f"Git error: {result.stderr[-300:]}\n"
-                f"PyPI error: {result2.stderr[-300:]}"
+                f"Diag: {_diag_info}\n"
+                f"Git stderr:\n{result.stderr[-2000:]}\n"
+                f"PyPI stderr:\n{result2.stderr[-2000:]}"
             )
 
     importlib.invalidate_caches()