build(cuda): ship a CUDA installer variant with bundled runtime DLLs

The default build (default = candle-cuda) load-time-imports the CUDA runtime,
so it crashes on any machine without the toolkit. Add a CUDA release variant
that bundles the redistributable runtime DLLs (cudart/cublas/cublasLt/curand,
verified via dumpbin) so it runs on any NVIDIA machine with just a driver.

- tauri.cuda.conf.json: a 'tauri build --config' overlay (CPU release
  untouched) that points the updater at latest-cuda.json so a CUDA install
  never self-updates into the CPU build
- windows/cuda-hooks.nsh: NSIS hook embeds the DLLs via File and extracts them
  next to phokus.exe, where the Windows loader searches
- build:app:cuda script; cuda-redist/ gitignored (DLLs copied from the toolkit
  locally), with a negation so the overlay stays tracked

src-tauri/tauri.cuda.conf.json

@@ -0,0 +1,17 @@
+{
+  "$schema": "https://schema.tauri.app/config/2",
+  "plugins": {
+    "updater": {
+      "endpoints": [
+        "https://github.com/JezzWTF/phokus/releases/latest/download/latest-cuda.json"
+      ]
+    }
+  },
+  "bundle": {
+    "windows": {
+      "nsis": {
+        "installerHooks": "./windows/cuda-hooks.nsh"
+      }
+    }
+  }
+}

src-tauri/windows/cuda-hooks.nsh

@@ -0,0 +1,35 @@
+; NSIS hooks for the CUDA build variant.
+;
+; The CUDA exe load-time-imports the CUDA runtime DLLs, so they must sit next
+; to phokus.exe ($INSTDIR) where the Windows loader searches. Rather than
+; bundling them as Tauri "resources" (which land in $INSTDIR\resources\) and
+; then relocating them — a move that proved unreliable in the elevated/silent
+; installer (CopyFiles via SHFileOperation, and even Rename, failed) — we embed
+; them directly into the installer with NSIS `File` and extract them straight
+; to $INSTDIR. `File` is the standard, reliable install primitive.
+;
+; This hook is !included from its original location, and ${__FILEDIR__} in this
+; top-level !define expands at *include* time to this file's own directory,
+; src-tauri\windows. cuda-redist is its sibling under src-tauri, so one level
+; up. (Caveat that bit us: ${__FILEDIR__} used inline *inside the macro* would
+; instead expand at insertion time to Tauri's generated nsis dir — hence the
+; !define here.) The 4 DLLs must be present in src-tauri\cuda-redist at build
+; time (see RELEASE_PLAN.md).
+
+!define CUDA_REDIST "${__FILEDIR__}\..\cuda-redist"
+
+!macro NSIS_HOOK_POSTINSTALL
+  DetailPrint "Phokus: installing bundled CUDA runtime libraries..."
+  SetOutPath "$INSTDIR"
+  File "${CUDA_REDIST}\cudart64_12.dll"
+  File "${CUDA_REDIST}\cublas64_12.dll"
+  File "${CUDA_REDIST}\cublasLt64_12.dll"
+  File "${CUDA_REDIST}\curand64_10.dll"
+!macroend
+
+!macro NSIS_HOOK_POSTUNINSTALL
+  Delete "$INSTDIR\cudart64_12.dll"
+  Delete "$INSTDIR\cublas64_12.dll"
+  Delete "$INSTDIR\cublasLt64_12.dll"
+  Delete "$INSTDIR\curand64_10.dll"
+!macroend