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/c_import.zig

@@ -235,6 +235,58 @@ pub fn processCImport(
 // Native C compilation (compile to .o, not LLVM module)
 // ---------------------------------------------------------------------------
 
+// ── #foreign ref validation ───────────────────────────────────────────
+
+fn collectForeignRefTargets(valid: *std.StringHashMap(void), decls: []const *Node) !void {
+    for (decls) |d| {
+        switch (d.data) {
+            .library_decl => |ld| try valid.put(ld.name, {}),
+            .namespace_decl => |ns| {
+                // A NAMED `#import c` unit lowers to a namespace wrapping
+                // its c_import_decl — the namespace name is the unit ref.
+                for (ns.decls) |nd| {
+                    if (nd.data == .c_import_decl) {
+                        try valid.put(ns.name, {});
+                        break;
+                    }
+                }
+                try collectForeignRefTargets(valid, ns.decls);
+            },
+            else => {},
+        }
+    }
+}
+
+fn checkForeignRefs(valid: *const std.StringHashMap(void), decls: []const *Node, diags: *@import("errors.zig").DiagnosticList) void {
+    for (decls) |d| {
+        switch (d.data) {
+            .fn_decl => |fd| {
+                if (fd.body.data != .foreign_expr) continue;
+                const ref = fd.body.data.foreign_expr.library_ref orelse continue;
+                if (!valid.contains(ref)) {
+                    diags.addFmt(.err, d.span, "#foreign library '{s}' is not declared; expected a #library constant or a named '#import c' unit", .{ref});
+                }
+            },
+            .namespace_decl => |ns| checkForeignRefs(valid, ns.decls, diags),
+            else => {},
+        }
+    }
+}
+
+/// Validate every `#foreign <ref>` in the merged program: the ref must
+/// name a `#library` constant or a NAMED `#import c` unit. Refs are
+/// author-side module-local names, but modules merge flat or
+/// namespaced into one tree, so existence is checked program-wide.
+/// Decls synthesized from `#include` headers carry no ref and are
+/// exempt.
+pub fn validateForeignRefs(allocator: std.mem.Allocator, root: *const Node, diags: *@import("errors.zig").DiagnosticList) !void {
+    if (root.data != .root) return;
+    var valid = std.StringHashMap(void).init(allocator);
+    defer valid.deinit();
+    try collectForeignRefTargets(&valid, root.data.root.decls);
+    checkForeignRefs(&valid, root.data.root.decls, diags);
+}
+
 /// A cached entry must at least LOOK like an object file (Mach-O or
 /// ELF magic) — a truncated or garbage entry falls back to a fresh
 /// compile instead of poisoning the link with an opaque error.