feat(topdata): materialize Git-LFS asset bytes inside Crucible before packing

Adds materializeAssetLFS + hasLFSPointerStubs helpers to top_package.go.
Called at the top of packageBuiltTopData so bare CI checkouts never ship
~130-byte LFS pointer stubs. Opt-out via CRUCIBLE_SKIP_LFS env or the new
--skip-lfs flag on build-topdata / build-top-package.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/topdata/lfs_materialize_test.go

@@ -0,0 +1,29 @@
+package topdata
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestHasLFSPointerStubsDetectsStub(t *testing.T) {
+	root := t.TempDir()
+	if err := os.MkdirAll(filepath.Join(root, "a"), 0o755); err != nil {
+		t.Fatalf("mkdir: %v", err)
+	}
+	stub := "version https://git-lfs.github.com/spec/v1\noid sha256:abc\nsize 12345\n"
+	writeFile(t, filepath.Join(root, "a", "model.mdl"), stub)
+	writeFile(t, filepath.Join(root, "a", "real.txt"), "not a pointer, real bytes here")
+
+	if !hasLFSPointerStubs(root) {
+		t.Fatal("expected stub detection to be true")
+	}
+}
+
+func TestHasLFSPointerStubsNoStub(t *testing.T) {
+	root := t.TempDir()
+	writeFile(t, filepath.Join(root, "real.mdl"), "binary-ish content without a pointer header")
+	if hasLFSPointerStubs(root) {
+		t.Fatal("expected no stub detected")
+	}
+}