Fix Windows Rust CI cache: full target, Cargo registry, explicit save.

Cache the entire windows-setup target tree plus ~/.cargo; split restore/save; skip cargo build when all release binaries are present.

Co-authored-by: Cursor <cursoragent@cursor.com>

.github/actions/save-windows-rust-cache/action.yml

@@ -0,0 +1,14 @@
+name: Save Windows Rust build cache
+description: Save Cargo registry, git checkouts, and the windows-setup target directory
+
+runs:
+  using: composite
+  steps:
+    - uses: actions/cache/save@v5
+      continue-on-error: true
+      with:
+        key: windows-rust-${{ runner.os }}-v2-${{ hashFiles('app/desktop/windows-setup/Cargo.lock', 'app/desktop/windows-setup/Cargo.toml', 'app/desktop/windows-setup/**/src/**', 'app/desktop/windows-setup/**/build.rs') }}
+        path: |
+          ~/.cargo/registry
+          ~/.cargo/git
+          app/desktop/windows-setup/target