fix(test): resolve test runner thread stack overflow and JSON path formatting

Fix two test failures affecting CI and local test runs:

1. Cross-platform test runner thread stack overflow:
   In debug builds (opt-level = 0), `app.view(wid)` constructs the entire Iced UI
   widget tree (ribbon, tool panels, palettes, command line, status bar).
   Because libtest spawns test worker threads without setting an explicit stack
   size, the default thread stack allocated by the OS (2MB on Linux glibc, 1-2MB
   on Windows) overflows during recursive un-inlined view construction in
   `app::automation::tests::test_pline_line_then_arc`.
   - Set `[env] RUST_MIN_STACK = "8388608"` in `.cargo/config.toml` so `cargo test`
     allocates 8MB per test thread across all platforms (Linux CI, macOS, Windows).
   - Wrapped `test_pline_line_then_arc` in an explicit 8MB thread via
     `std::thread::Builder` to guarantee immunity even when invoked without env vars.

2. Windows temporary path formatting in JSON payload:
   In `app::update::page_setup_import::tests::psetupin_imports_named_setups_from_a_saved_drawing`,
   interpolating `source.display()` directly into a raw JSON string resulted in
   unescaped backslashes from Windows paths, causing JSON deserialization to fail.
   Replaced backslashes with forward slashes to ensure cross-platform valid JSON strings.

src/app/update/page_setup_import.rs

@@ -260,7 +260,8 @@ mod tests {
             app.tabs[i].scene.page_setup_save("Site", ps.clone());
             ps.paper_size = "ISO_A2_(420.00_x_594.00_MM)".into();
             app.tabs[i].scene.page_setup_save("Overview", ps);
-            let save = format!(r#"{{"op":"save","path":"{}"}}"#, source.display());
+            let p = source.to_string_lossy().replace('\\', "/");
+            let save = format!(r#"{{"op":"save","path":"{p}"}}"#);
             let reply = app.automation_op(&save);
             assert_eq!(reply["ok"], true, "{reply}");
         }