feat(#141): export FileCoveredByDocMap helper in review/docmap.go

Adds FileCoveredByDocMap(cfg *DocMapConfig, file string) bool — a thin wrapper
over the existing unexported mappingMatches that lets cmd/ check per-file docmap
coverage without duplicating glob logic.

Also adds unit tests covering matched globs, non-matching paths, empty file,
and empty config.

review/docmap_test.go

@@ -436,3 +436,52 @@ func writeTempYAML(t *testing.T, content string) string {
 	}
 	return filepath.Clean(f.Name())
 }
+
+// ============================================================
+// FileCoveredByDocMap
+// ============================================================
+
+func TestFileCoveredByDocMap(t *testing.T) {
+	cfg := &DocMapConfig{
+		Mappings: []DocMapping{
+			{
+				Paths: []string{"lib/foo/**", "lib/bar/*.go"},
+				Docs:  []string{"docs/foo.md"},
+			},
+			{
+				Paths: []string{"cmd/**"},
+				Docs:  []string{"docs/cmd.md"},
+			},
+		},
+	}
+
+	cases := []struct {
+		file    string
+		covered bool
+	}{
+		{"lib/foo/baz.ex", true},
+		{"lib/foo/sub/deep.ex", true},
+		{"lib/bar/util.go", true},
+		{"lib/bar/sub/util.go", false}, // *.go only matches one level
+		{"cmd/main.go", true},
+		{"cmd/sub/main.go", true},
+		{"internal/secret.go", false},
+		{"", false},
+	}
+
+	for _, tc := range cases {
+		t.Run(tc.file, func(t *testing.T) {
+			got := FileCoveredByDocMap(cfg, tc.file)
+			if got != tc.covered {
+				t.Errorf("FileCoveredByDocMap(%q) = %v, want %v", tc.file, got, tc.covered)
+			}
+		})
+	}
+}
+
+func TestFileCoveredByDocMap_EmptyConfig(t *testing.T) {
+	cfg := &DocMapConfig{}
+	if FileCoveredByDocMap(cfg, "lib/foo/bar.go") {
+		t.Error("expected false for empty config, got true")
+	}
+}