feat(C3.1): #foreign refs are validated — must name a #library or a named #import c unit

validateForeignRefs walks the merged tree (libraries + named c units,
nested namespaces included) and diagnoses any #foreign whose ref names
neither — a typo'd ref previously compiled and resolved silently
through whatever image carried the symbol. Decls synthesized from
#include headers carry no ref and are exempt. Flips the C0.2b pin;
zero collateral across the 608 other examples.

src/core.zig

@@ -298,6 +298,13 @@ pub const Compilation = struct {
     /// Lower the parsed AST to the sx IR module (shadow pipeline).
     pub fn lowerToIR(self: *Compilation) !ir.Module {
         const root = self.resolved_root orelse self.root orelse return ir.Module.init(self.allocator);
+
+        // Every `#foreign <ref>` must name a #library constant or a named
+        // `#import c` unit — a typo'd ref otherwise resolves silently
+        // through whatever image happens to carry the symbol.
+        try c_import.validateForeignRefs(self.allocator, root, &self.diagnostics);
+        if (self.diagnostics.hasErrors()) return error.CompileError;
+
         var module = ir.Module.init(self.allocator);
         //TODO: find a better place for this
         if (self.target_config.isWasm32()) {