wip(E4): partial source-pin + non-transitive flip [stdlib E4 attempt-1 WIP checkpoint]

Incomplete WIP from a worker killed at the 55-min wall (large blast radius:
core source-pin + ~8 example migrations + ~10 library module migrations).
Committed so the resumed session continues on a clean tree. May not build.

src/imports.zig

@@ -661,14 +661,39 @@ fn reportDuplicateName(diagnostics: ?*errors.DiagnosticList, added: bool, name:
 fn stampFnBodySource(decl: *Node, file_path: []const u8) void {
     switch (decl.data) {
         .fn_decl => |fd| fd.body.source_file = file_path,
+        .struct_decl => |sd| stampStructMethodSources(sd, file_path),
+        // A parameterized protocol is instantiated cross-module; record its
+        // defining path so the instantiation resolves method-signature types in
+        // this module (E4).
+        .protocol_decl => decl.data.protocol_decl.source_file = file_path,
         .const_decl => |cd| switch (cd.value.data) {
             .fn_decl => |fd| fd.body.source_file = file_path,
+            // `List :: struct { … append :: (…) { … } }` — the methods of a
+            // (possibly generic) struct are monomorphized in their template's
+            // OWN module (issue 0106 + the E4 instantiation source-pin), so their
+            // bodies need the defining path stamped just like a top-level fn.
+            .struct_decl => |sd| stampStructMethodSources(sd, file_path),
+            .protocol_decl => cd.value.data.protocol_decl.source_file = file_path,
             else => {},
         },
         else => {},
     }
 }
 
+/// Stamp the defining module path onto every method (and struct-level fn
+/// constant) body of a struct decl, so a generic-struct method monomorphized at
+/// a cross-module call site still pins to the module that declares it.
+fn stampStructMethodSources(sd: ast.StructDecl, file_path: []const u8) void {
+    for (sd.methods) |m| {
+        if (m.data == .fn_decl) m.data.fn_decl.body.source_file = file_path;
+    }
+    for (sd.constants) |c| {
+        if (c.data == .const_decl and c.data.const_decl.value.data == .fn_decl) {
+            c.data.const_decl.value.data.fn_decl.body.source_file = file_path;
+        }
+    }
+}
+
 /// `reportDuplicateName` keyed off a node whose `declName()` carries the name
 /// (the regular authored-decl sites; an `import_decl` has no `declName`, so a
 /// namespace alias must use `reportDuplicateName` with the alias directly).