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/ast.zig

@@ -783,6 +783,11 @@ pub const ProtocolDecl = struct {
     /// True when the declared NAME was a backtick raw identifier — exempt from
     /// the reserved-type-name decl check (issue 0089).
     is_raw: bool = false,
+    /// Defining module path (stamped by `resolveImports`), so a parameterized
+    /// protocol instantiated cross-module resolves its method signature types in
+    /// the module that declares it (E4 — the protocol analog of
+    /// `StructTemplate.source_file`). Null for a synthesized/sourceless decl.
+    source_file: ?[]const u8 = null,
 };
 
 pub const ForeignRuntime = enum {