source: abort guard compares to the originating screen, not 0

A script opened from the dashboard (screen_idx 4) aborted after its first
line: the guard treated any non-console screen as 'an interactive command
opened a screen'. Record the screen the source started from and abort only
when a sourced line navigates away from it (what a bare interactive command
does). Now 'o' from the dashboard runs the whole script in place — the
dashboard populates live behind the global Computing overlay — while a bare
connect/explore inside a script still aborts. Batch unaffected (BootDispatch
pins screen 0, so origin 0).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/tui/shell.cpp

@@ -296,6 +296,7 @@ void Tui::Source(const std::string &filename) {
     loading_executed = 0;
     loading_lineno = 0;
     loading_prev_in_source = in_source;
+    source_origin_screen = screen_idx;  // a sourced line that leaves this screen aborts
     in_source = true;
     loading = true;
 
@@ -346,10 +347,10 @@ void Tui::ProcessNextSourceLine() {
         Submit();
         ++loading_executed;
 
-        if (screen_idx != 0) {
+        if (screen_idx != source_origin_screen) {
             Print("source: line " + std::to_string(loading_lineno)
                   + " is interactive (would open a screen) — aborting.");
-            screen_idx = 0;
+            screen_idx = source_origin_screen;
             loading.store(false);
             tick_in_flight.store(false);
             in_source = loading_prev_in_source;