cuda-dll: make the Windows CUDA build work as shipped — MSVC host flags, CUDA_PATH defaults, POSIX shim in kernel test (#158, #157)

* cuda-dll: fix Windows build — MSVC host flags, CUDA_PATH default, POSIX setenv shim in the kernel test (#157)

First hardware validation of the #131 CUDA_DLL path (RTX PRO 6000 Blackwell
sm_120, MSVC 14.44 + CUDA 13.2, MSYS2 UCRT64 host build) found three blockers
that made 'make cuda-dll' unbuildable as shipped:

- NVCCFLAGS passed GCC-style -Xcompiler=-Wall,-Wextra to the MSVC host
  compiler (hard error D8021). Use -Xcompiler=-W3 on Windows — dash form,
  since MSYS make mangles /W3 into a filesystem path.
- NVCC defaulted to $(CUDA_HOME)/bin/nvcc with CUDA_HOME=/usr/local/cuda;
  on Windows default CUDA_HOME from the installer's CUDA_PATH and NVCC to
  plain 'nvcc' from PATH (CUDA_PATH contains spaces, which the unquoted
  recipe checks cannot survive; an MSVC PATH environment is already required).
- tests/test_backend_cuda.cu used POSIX setenv/unsetenv (undefined under
  MSVC); add a two-line _putenv_s shim.

Also corrects the stale '11 API symbols' comment (the header exports 15 and
backend_loader.c resolves all 15).

Validated: make cuda-dll (stock flags) + make glm CUDA_DLL=1 ARCH=native →
[CUDA] device init on sm_120, tiny oracle TF 32/32 + greedy 20/20, kernel
suite 'q8/q4/q2/f32 correctness ok', graceful no-dll fallback, plain build
byte-identical CPU behavior.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* cuda: fix heap corruption in expert_host_release on Windows (CUDA_RELEASE_HOST=1)

expert_host_release() freed the expert slab with plain free(), but the slab
is posix_memalign'd — which compat.h maps to _aligned_malloc on Windows, so
free() corrupts the CRT heap: instant 0xC0000374 crash on the first released
expert. This is the exact pattern the compat.h audit fixed at the original
expert_load site ("l'unico sito che libera memoria aligned e' free(s->slab)");
this call site was added later and reintroduced it. compat_aligned_free is
plain free on POSIX, so non-Windows behavior is unchanged. fslab stays plain
free (malloc/falloc on the CPU path).

Found running the VRAM expert tier on real hardware (80 GB resident on an
RTX PRO 6000, CUDA_RELEASE_HOST=1 to avoid 80 GB of host double-residency —
reproducible crash before, clean generation after).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Co-authored-by: lEWFkRAD <186512915+lEWFkRAD@users.noreply.github.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: JustVugg <JustVugg@users.noreply.github.com>

c/Makefile

@@ -105,10 +105,30 @@ INSTALL    ?= install
 #                   clean. See backend_loader.c and README "cuda-dll" below.
 CUDA      ?= 0
 CUDA_DLL  ?= 0
+ifneq ($(IS_WIN),)
+# the CUDA installer sets CUDA_PATH system-wide (e.g. C:\Program Files\NVIDIA
+# GPU Computing Toolkit\CUDA\v13.2); fall back to it before the POSIX default.
+# NVCC defaults to plain `nvcc` from PATH: CUDA_PATH contains spaces, which the
+# unquoted recipe checks cannot survive — and the cuda-dll recipe already
+# requires an MSVC environment (vcvars64) on PATH, so requiring the CUDA bin
+# directory too is symmetric. The installer adds it by default.
+CUDA_HOME ?= $(subst \,/,$(CUDA_PATH))
+NVCC      ?= nvcc
+else
 CUDA_HOME ?= /usr/local/cuda
 NVCC      ?= $(CUDA_HOME)/bin/nvcc
+endif
 CUDA_ARCH ?= native
+ifneq ($(IS_WIN),)
+# nvcc's host compiler on Windows is MSVC cl.exe: the GCC-style
+# -Xcompiler=-Wall,-Wextra is rejected ("D8021 invalid numeric argument
+# '/Wextra'"), which made `make cuda-dll` unbuildable as shipped. -W3 is
+# the MSVC warning level (dash form, NOT /W3: MSYS make would mangle the
+# slash-form into a filesystem path).
+NVCCFLAGS ?= -O3 -std=c++17 -arch=$(CUDA_ARCH) -Xcompiler=-W3
+else
 NVCCFLAGS ?= -O3 -std=c++17 -arch=$(CUDA_ARCH) -Xcompiler=-Wall,-Wextra
+endif
 ifeq ($(IS_WIN),)
 PYTHON    ?= python3
 else
@@ -169,7 +189,7 @@ backend_loader.o: backend_loader.c backend_cuda.h compat.h
 # compiler, required by nvcc on Windows) into coli_cuda.dll. Run this from a
 # shell that has the MSVC environment set (e.g. after vcvars64.bat, or from a
 # "x64 Native Tools Command Prompt"). COLI_CUDA_BUILDING_DLL enables
-# __declspec(dllexport) so the 11 API symbols are exported.
+# __declspec(dllexport) so the 15 API symbols are exported.
 cuda-dll: backend_cuda.cu backend_cuda.h
 	@command -v $(NVCC) >/dev/null 2>&1 || { echo "nvcc not found: set CUDA_HOME or NVCC" >&2; exit 1; }
 	@command -v cl >/dev/null 2>&1 || { echo "cl.exe (MSVC) not in PATH — run vcvars64.bat first" >&2; exit 1; }