fix(palabras): panel/inspector — tab válida, alt sin base64, spacing sin fuga, draft sin doble commit

Cuatro defectos confirmados por el barrido:

- Panel (#5): tras clickear una tab (tabAuto=false), cambiar a un bloque sin
  Contenido dejaba la tab en 'contenido' (trigger no renderiza) → panel en blanco
  sin tab activa. El efecto de auto-landing ahora corrige una tab INVÁLIDA aunque
  tabAuto=false (cae a la 1a disponible). Verificado Playwright: párrafo → Diseño.

- Inspector (#7): el draft de `src` derivaba `alt` de split('/')/split('.') →
  basura base64 para data: URLs. Ahora se salta la derivación para data:.

- Spacing (#10): el override de enlace (localUnlink) se reseteaba por
  JSON.stringify(value); dos bloques sin spacing serializan igual ('null') → el
  override se filtraba entre ellos. Ahora resetea por identidad de bloque
  (resetKey=api.activeBlockId), cableado en panel + inspector.

- Draft-field (#12): Enter llamaba confirm() y luego blur() que re-dispara
  onblur=confirm antes de que `value` se propague → doble onCommit → 2 entradas
  de historial. Guard de reentrada por el flag `editing`.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/uix/eidos/components/palabras/palabras-draft-field.svelte

@@ -46,8 +46,12 @@
 	const dirty = $derived(editing && draft !== value);
 
 	function confirm() {
-		if (draft !== value) onCommit(draft);
+		// Guard re-entry: Enter calls confirm() and then blur(), and blur re-fires
+		// onblur=confirm SYNCHRONOUSLY before the external `value` flushes — without
+		// this guard the same draft commits twice (two history entries for one edit).
+		if (!editing) return;
 		editing = false;
+		if (draft !== value) onCommit(draft);
 	}
 	function revert() {
 		draft = value;

src/uix/eidos/components/palabras/palabras-element-inspector.svelte

@@ -311,7 +311,10 @@
 		const safe = sanitizeImageSrc(draft);
 		if (safe === undefined) return;
 		write(field.key, safe);
-		if (field.key === 'src' && safe && valueOf('alt') == null) {
+		// Derive an alt guess from the filename — but ONLY for a real URL. A data:
+		// URL carries no filename (split('/')/split('.') would yield a base64 chunk),
+		// so skip it and leave alt empty for the author to fill.
+		if (field.key === 'src' && safe && !/^data:/i.test(safe) && valueOf('alt') == null) {
 			const guess = safe.split('/').pop()?.split('.')[0];
 			if (guess) write('alt', guess);
 		}
@@ -477,6 +480,7 @@
 				label={ts(field.label)}
 				value={effective(field) as SpacingValue | undefined}
 				{disabled}
+				resetKey={api.activeBlockId}
 				onValueChange={(s) => commit(field, s)}
 			/>
 		</div>

src/uix/eidos/components/palabras/palabras-panel.svelte

@@ -335,9 +335,22 @@
 	// the user picks a tab, `tabAuto` turns off and their choice persists.
 	let tabAuto = $state(true);
 	$effect(() => {
-		if (!tabAuto) return;
+		const has = (t: string) =>
+			t === 'contenido'
+				? contenidoFields.length > 0
+				: t === 'diseño'
+					? disenoSections.length > 0
+					: avanzadoFields.length > 0;
 		const next = contenidoFields.length ? 'contenido' : disenoSections.length ? 'diseño' : 'avanzado';
-		if (next !== tab) tab = next;
+		if (tabAuto) {
+			// Auto mode: keep landing on the first non-empty tab.
+			if (next !== tab) tab = next;
+		} else if (!has(tab)) {
+			// User picked a tab, then switched to a block that lacks it (only the
+			// Contenido trigger is conditional) — the tab would be lit-less and the
+			// panel blank. Fall to the first available tab even in manual mode.
+			tab = next;
+		}
 	});
 
 	// ── Reset to defaults — per Diseño accordion section + per whole block. Clears
@@ -1025,6 +1038,7 @@
 				label={ts(field.label)}
 				value={effective(field) as SpacingValue | undefined}
 				{disabled}
+				resetKey={api.activeBlockId}
 				onValueChange={(s) => commit(field, s)}
 			/>
 		</div>

src/uix/eidos/components/palabras/palabras-spacing.svelte

@@ -29,6 +29,7 @@
 		label,
 		value,
 		disabled = false,
+		resetKey,
 		onValueChange
 	}: {
 		/** Field label — the control OWNS its header row (label + link toggle). */
@@ -36,6 +37,10 @@
 		/** Current spacing (raw px numbers, per axis and/or per side). */
 		value: PalabrasSpacing | undefined;
 		disabled?: boolean;
+		/** Block identity — resets the empty-mode link override on a block switch.
+		 *  Two empty-spacing blocks serialize identically, so the override can't be
+		 *  reset from `value` alone; the caller passes the active block id. */
+		resetKey?: string | number | null;
 		/** Fired with the FULL next spacing, or `undefined` when every side is cleared. */
 		onValueChange: (value: PalabrasSpacing | undefined) => void;
 	} = $props();
@@ -65,14 +70,14 @@
 	);
 	const linked = $derived(isEmpty ? !localUnlink : dataLinked);
 
-	// Drop the empty-mode override whenever the bound value changes, so a new block
-	// starts linked and real data drives the state. `lastKey` is non-reactive (no loop:
-	// the effect reads `value`, not `localUnlink`).
-	let lastKey = '';
+	// Drop the empty-mode override on a BLOCK SWITCH (identity change), so a new block
+	// starts linked. Keying on `value` (as before) can't distinguish two empty-spacing
+	// blocks — both serialize to "null" — so the override would leak across them.
+	// `lastKey` is non-reactive (no loop: the effect reads `resetKey`, not `localUnlink`).
+	let lastKey: string | number | null | undefined;
 	$effect(() => {
-		const key = JSON.stringify(value ?? null);
-		if (key !== lastKey) {
-			lastKey = key;
+		if (resetKey !== lastKey) {
+			lastKey = resetKey;
 			localUnlink = false;
 		}
 	});