Fix CUDA detection on Windows: cuda_binary()/cuda_linkage() always returned False

Both coli's cuda_binary() and doctor.py's cuda_linkage() detect CUDA support
by running `ldd` on the engine binary and looking for a linked libcudart —
but that's Linux-only (cuda_binary() checks sys.platform != "linux", and
cuda_linkage() checks os.name != "posix", both short-circuiting to False on
win32). Windows CUDA_DLL=1 builds never link libcudart at all: glm.exe loads
coli_cuda.dll dynamically via LoadLibrary at startup (backend_loader.c), so
there's no import-table entry for ldd/dumpbin to find in the first place.

The practical effect: `coli doctor` always reported "NVIDIA GPU detected but
the engine is CPU-only" on Windows, and `coli run/chat/serve --gpu ...`
always hard-exited with "--gpu needs the CUDA build" — even on a correctly
built CUDA_DLL=1 binary with coli_cuda.dll sitting right next to glm.exe.

Fix: on win32, detect a COLI_CUDA build by scanning glm.exe for the marker
string "[CUDA] mode: routed experts", which only exists in code compiled
under #ifdef COLI_CUDA (see glm.c's cuda init block), then confirm
coli_cuda.dll actually sits next to the binary — mirroring the Linux
"linked but missing" distinction. Linux/macOS detection is unchanged.

Verified on Windows 11 with mingw-w64 GCC 16.1.0 + CUDA 13.2 + RTX 5080:
`coli doctor` now reports "CUDA engine and devices are available", and
`coli run --gpu 0 --vram 8` populates VRAM (confirmed via the engine's own
"[CUDA] resident set: N tensors, X GB VRAM" runtime log) instead of exiting.

c/coli

@@ -141,12 +141,24 @@ def need_model(model):
         sys.exit(f"{C.yel}engine is not built.{C.r} Run: coli build")
 
 def cuda_binary():
-    if not os.path.exists(GLM) or sys.platform != "linux": return False
-    try:
-        linked=subprocess.run(["ldd",GLM],capture_output=True,text=True,timeout=3)
-        return any("libcudart" in line and "not found" not in line
-                   for line in linked.stdout.splitlines())
-    except (OSError,subprocess.SubprocessError): return False
+    if not os.path.exists(GLM): return False
+    if sys.platform == "linux":
+        try:
+            linked=subprocess.run(["ldd",GLM],capture_output=True,text=True,timeout=3)
+            return any("libcudart" in line and "not found" not in line
+                       for line in linked.stdout.splitlines())
+        except (OSError,subprocess.SubprocessError): return False
+    if sys.platform == "win32":
+        # Windows CUDA_DLL=1 builds never link libcudart directly: glm.exe loads
+        # coli_cuda.dll at runtime via LoadLibrary (backend_loader.c), so there's no
+        # import-table entry for ldd/dumpbin to see. Detect the COLI_CUDA build via a
+        # marker string baked into glm.c's #ifdef COLI_CUDA block instead, and require
+        # coli_cuda.dll to actually sit next to glm.exe (else CUDA init fails at startup).
+        try:
+            with open(GLM,"rb") as f: built=b"[CUDA] mode: routed experts" in f.read()
+        except OSError: return False
+        return built and os.path.exists(os.path.join(os.path.dirname(GLM),"coli_cuda.dll"))
+    return False
 
 def resource_request(a, env):
     ctx=a.ctx or int(env.get("CTX",4096))