Add focused changed-path test wrapper

tests/changed_tests.bats

@@ -0,0 +1,27 @@
+#!/usr/bin/env bats
+# Seam: the changed-test wrapper selects only the tests associated with
+# staged paths and refuses to silently run the entire suite.
+
+load helpers
+
+@test "changed-test wrapper is executable and uses staged diff by default" {
+    [ -x "$REPO_ROOT/scripts/test-changed.sh" ]
+    run grep -F 'git diff --cached' "$REPO_ROOT/scripts/test-changed.sh"
+    [ "$status" -eq 0 ]
+}
+
+@test "changed-test wrapper maps a compose change to compose and environment tests" {
+    run bash "$REPO_ROOT/scripts/test-changed.sh" --list --paths dankserver/compose.yaml
+    [ "$status" -eq 0 ]
+    [[ "$output" == *"tests/compose_validity.bats"* ]]
+    [[ "$output" == *"tests/env_consistency.bats"* ]]
+    [[ "$output" != *"tests/homepage_pages.bats"* ]]
+}
+
+@test "changed-test wrapper maps a service script to its focused config tests" {
+    run bash "$REPO_ROOT/scripts/test-changed.sh" --list --paths scripts/serve-frank-small.sh
+    [ "$status" -eq 0 ]
+    [[ "$output" == *"tests/scripts_syntax.bats"* ]]
+    [[ "$output" == *"tests/frank_small_config.bats"* ]]
+    [[ "$output" != *"tests/frank_large_config.bats"* ]]
+}