ir: whole-program passes pin the source context per decl (fix 0122)

convergeClosureShapeSets, checkErrorFlow, and the unknown-type loop ran
under whatever current_source_file the previous phase left behind —
closure-literal annotations resolved (and reject/unknown-type
diagnostics rendered) against an arbitrary module. Latent while std.sx
was a single file (the ambient happened to be the main file); the
re-export facade restructure exposed it. Each walk now pins
setCurrentSourceFile per decl / per fn (body.source_file is already
stamped by resolveImports). Coverage: examples 0129/1047/1049/1052/
1053/1056 against the facade std.sx. Gates: zbt 426/426, suite 588/588.

src/ir/error_analysis.zig

@@ -191,8 +191,16 @@ pub const ErrorAnalysis = struct {
     /// by all occurrences of its value-signature shape. A `try slot(x)` against
     /// any matching-shape slot then widens against this union.
     pub fn convergeClosureShapeSets(self: ErrorAnalysis) void {
+        // Pin the visibility context to each fn's DEFINING module
+        // (body.source_file, stamped by resolveImports) — a closure literal's
+        // param/return annotations must resolve where the fn is written, not
+        // against whatever module the previous pipeline phase happened to
+        // leave as the ambient context (issue 0122).
+        const saved = self.l.current_source_file;
+        defer self.l.setCurrentSourceFile(saved);
         var it = self.l.program_index.fn_ast_map.iterator();
         while (it.next()) |e| {
+            self.l.setCurrentSourceFile(e.value_ptr.*.body.source_file orelse saved);
             self.collectClosureShapes(e.value_ptr.*.body);
         }
     }