fix: CUDA/GPU detection for vLLM and llama.cpp in Docker (#479)

Two bugs caused GPU inference to silently fall back to CPU inside the
Odysseus Docker container even when the GPU was correctly passed through.

## entrypoint.sh — CUDA_HOME detection only covered CUDA 13.x wheels

The nvcc glob only searched
vidia/cu13, which matches the

vidia-nvcc-cu13 pip wheel layout. CUDA 12.x wheels install nvcc to

vidia/cuda_nvcc/bin/nvcc (nvidia-cuda-nvcc-cu12) or
vidia/cu12
(nvidia-nvcc-cu12) — completely different paths. The glob found nothing,
so CUDA_HOME was never set.

Worse, VLLM_USE_FLASHINFER_SAMPLER=0 was inside the same if-block, so it
was never set either. vLLM then tried to JIT-compile the FlashInfer
sampler at startup, failed with 'Could not find nvcc', and crashed — even
though the GPU was fully visible to the container.

Fix: expand the search to also check nvidia/cu12 and nvidia/cuda_nvcc.
Move VLLM_USE_FLASHINFER_SAMPLER=0 to an unconditional export after the
loop (it is sampler-only, no impact on the attention path, and the correct
setting for any container where CUDA headers may be incomplete).

## cookbook_routes.py — llama.cpp Linux source build silently fell back to CPU

The cmake invocation was:
  cmake -B build -DGGML_CUDA=ON 2>/dev/null || cmake -B build

2>/dev/null suppressed all configure errors. When nvcc is absent (the
slim base image has no CUDA toolkit — intentional), cmake fails silently,
then the || fallback re-runs without -DGGML_CUDA=ON. A CPU-only binary is
produced with no warning. Additionally, a stale CMakeCache.txt from the
failed CUDA attempt was reused (no rm -rf build), poisoning the next
configure run. The macOS branch already did rm -rf build for exactly this
reason; the Linux branch did not.

Fix: before cmake, detect pip-installed nvcc across the same three path
patterns as entrypoint.sh and expose it via CUDA_HOME/PATH. If nvcc is
found, run a clean CUDA build with full error visibility. If not, fall
back to a CPU build with an explicit warning telling the user how to get
a GPU build (install vLLM via Cookbook -> Dependencies, which brings the
CUDA wheels including nvcc, then re-launch).

## .env.example — document Windows COMPOSE_FILE separator

Added a comment showing the semicolon separator required on Windows
Docker Desktop alongside the existing colon-separator (Linux) example.

.env.example

@@ -137,6 +137,7 @@ SEARXNG_INSTANCE=http://localhost:8080
 # NVIDIA (requires nvidia-container-toolkit + `nvidia-ctk runtime
 # configure --runtime=docker` on the host):
 # COMPOSE_FILE=docker-compose.yml:docker/gpu.nvidia.yml
+# COMPOSE_FILE=docker-compose.yml;docker/gpu.nvidia.yml    #(Windows)
 #
 # AMD ROCm (requires ROCm drivers on the host):
 # COMPOSE_FILE=docker-compose.yml:docker/gpu.amd.yml