fix(buscar): correct the gaps between the filter row and the results

Issue 3 (partial): the results area had no top gap against the filter
row in one state and reused the horizontal constant for a vertical axis
in another. Applies the 3-tier scale properly -- row tier for
background-less placeholders, card tier for card states.

The rest of the app's spacing review is still outstanding.

lib/pantallas/pantalla_buscar.dart

@@ -543,8 +543,17 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
     if (estado.cargando) {
       // S5-R6: shimmer placeholders instead of a bare spinner, consistent
       // with the loading pattern used by the home grid.
+      // Issue 3 (feedback-pruebas): row tier (12), not card tier (16) --
+      // these are background-less row placeholders, same tier as the real
+      // results below; the top inset is the standard section gap rather
+      // than the horizontal constant reused for a vertical axis.
       return Padding(
-        padding: const EdgeInsets.all(PluriLayout.horizontal),
+        padding: const EdgeInsets.fromLTRB(
+          PluriLayout.rowHorizontal,
+          PluriLayout.sectionGap,
+          PluriLayout.rowHorizontal,
+          PluriLayout.rowHorizontal,
+        ),
         child: Column(
           crossAxisAlignment: CrossAxisAlignment.start,
           children: [
@@ -579,8 +588,17 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
       // across the app -- favorites, the discovery grid -- which this
       // item does not touch).
       final query = _controller.text.trim();
+      // Issue 3 (feedback-pruebas): this card-tier state had NO top gap at
+      // all against the filter row above it -- the standard section gap
+      // now matches the other two mutually-exclusive results-area states
+      // (loading, populated) above.
       return Padding(
-        padding: const EdgeInsets.symmetric(horizontal: PluriLayout.horizontal),
+        padding: const EdgeInsets.fromLTRB(
+          PluriLayout.horizontal,
+          PluriLayout.sectionGap,
+          PluriLayout.horizontal,
+          0,
+        ),
         child: _TarjetaSinResultados(
           titulo:
               sinFiltros
@@ -616,7 +634,17 @@ class _PantallaBuscarState extends State<PantallaBuscar> {
     return ListView.builder(
       shrinkWrap: true,
       physics: const NeverScrollableScrollPhysics(),
-      padding: const EdgeInsets.all(PluriLayout.horizontal),
+      // Issue 3 (feedback-pruebas): row tier (12), not card tier (16) --
+      // `FilaEmisoraPlana` rows are documented as "flat, background-less"
+      // (audit 6.5) but this padding never got updated to match when Tier 1
+      // introduced the 3-tier scale. The top inset is the standard section
+      // gap, not the horizontal constant reused for a vertical axis.
+      padding: const EdgeInsets.fromLTRB(
+        PluriLayout.rowHorizontal,
+        PluriLayout.sectionGap,
+        PluriLayout.rowHorizontal,
+        PluriLayout.rowHorizontal,
+      ),
       itemCount: total,
       itemBuilder: (context, i) {
         if (i >= resultados.length) {