Add AMD OpenCL kernel, runtime-loaded CUDA, mixed backend, portability

AMD GPU backend:
- Add the GCN-tuned equihash192_7.cl kernel (clearCounter/blake/round1..7/
  combine pipeline) and its host driver src/gpu_amd.rs. GpuSolver now dispatches
  AMD-vendor OpenCL devices to it and other devices to the existing kernel
  (force with ZCL_OPENCL_KERNEL=amd|legacy). Validated on an RX 9060 XT: GPU
  solutions match the CPU reference 1/1.
- Expose BatchHasher::midstate() for the kernel's ulong8 hashState arg.

Runtime-loaded GPU drivers (minimum host deps):
- dlopen libcuda / libnvidia-ml via libloading instead of linking them
  (src/dylib.rs macro; cuda.rs, nvml.rs, gpu_probe.rs). The binary now builds
  and starts on hosts without an NVIDIA driver and reports no CUDA devices
  gracefully; remove build.rs (its only job was linking those libs).
- Add Dockerfile.portable + build-portable.sh: build against Debian bullseye's
  glibc 2.31 for a binary that runs on older distros and drives both AMD
  (OpenCL) and NVIDIA (CUDA) cards. Document the build matrix in the README.

Mixed backend (default):
- Add --backend mixed (now the default): each card on its native backend
  (NVIDIA->CUDA, AMD/Intel->OpenCL), deduped so no card is mined twice.
  --devices indexes the unified list shown by --list-devices.

Misc:
- Stale-work timeout (--job-timeout) default 300s -> 600s (10 minutes).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mine.example.toml

@@ -23,9 +23,11 @@ user = "t1YourZClassicAddressHere.rig1"        # payout address / worker login
 # no-jackpot = true          # PPLNS
 
 # ── GPU backend ───────────────────────────────────────────────────────────────
-# backend      = "cuda"      # "cuda" or "opencl"
-# devices      = "all"       # "all", or a comma list e.g. "0,1"
-# force-opencl = false       # force OpenCL, disabling CUDA
+# backend      = "mixed"     # "mixed" (default: NVIDIA→CUDA, AMD/Intel→OpenCL),
+#                            # "opencl" (every card via OpenCL), or "cuda"
+# devices      = "all"       # "all", or a comma list e.g. "0,1" — in mixed mode
+#                            # these index the combined list from --list-devices
+# force-opencl = false       # force every card onto OpenCL, disabling CUDA
 
 # ── GPU tuning (clock/power changes need root) ────────────────────────────────
 # no-gpu-tune      = false