feat(views,surface-host): Extensions sidebar view — auto-expanded surfaces + tables

views (new feature):
- pure panel-stack reducer + thin generic ViewSidebar (dropdown picker + add/remove),
  switches on view KIND, never a surface id

Extensions view (composition root):
- folds frontend modules + backend surfaces into one "Extensions" view
- frontend module list AGGREGATED from each feature's public `manifest` export
  (can't drift); no per-module version (FE features are internal to dispatch-web)
- surfaces are AUTO-SUBSCRIBED on catalog + rendered expanded (no catalog buttons)

surface-host:
- consecutive `stat` fields coalesce into one aligned label/value table (StatTable)
- generic custom-field renderer: dispatch on rendererId === "table" → SurfaceTable
  (pure parseTablePayload), so a backend `custom`/table field renders generically
- shared presentational components/Table.svelte (used by both, neither feature
  depends on the other)

store:
- auto-subscribe every catalog entry, unsubscribe vanished ones, re-subscribe all
  on reconnect; expose all received specs via `surfaces` (drops single-selection)

backend-handoff: CR-1 — emit Loaded Extensions as a custom/table field; notes
what's already covered FE-side (renderer shipped, stat-table fallback works).

1 files changed, 54 insertions, 0 deletions

diff --git a/src/features/surface-host/logic/table.ts b/src/features/surface-host/logic/table.ts
new file mode 100644
index 0000000..027553c
--- /dev/null
+++ b/src/features/surface-host/logic/table.ts
@@ -0,0 +1,54 @@
+/**
+ * Pure parser for the `rendererId: "table"` custom-field payload.
+ *
+ * This is the FRONTEND-side renderer contract for tabular custom fields: a
+ * backend that wants a table emits a `custom` field with `rendererId: "table"`
+ * and a payload of `{ columns: string[]; rows: (string|number)[][] }`. Cells are
+ * coerced to strings. Anything that does not match the shape returns `null`, so
+ * the renderer gracefully skips it (never throws on hostile/partial data).
+ */
+
+export interface TableData {
+	readonly columns: readonly string[];
+	readonly rows: readonly (readonly string[])[];
+}
+
+function isStringArray(v: unknown): v is unknown[] {
+	return Array.isArray(v);
+}
+
+function coerceCell(v: unknown): string | null {
+	if (typeof v === "string") return v;
+	if (typeof v === "number" && Number.isFinite(v)) return String(v);
+	if (typeof v === "boolean") return String(v);
+	return null;
+}
+
+export function parseTablePayload(payload: unknown): TableData | null {
+	if (typeof payload !== "object" || payload === null) return null;
+	const obj = payload as Record<string, unknown>;
+
+	const rawColumns = obj.columns;
+	const rawRows = obj.rows;
+	if (!isStringArray(rawColumns) || !isStringArray(rawRows)) return null;
+
+	const columns: string[] = [];
+	for (const col of rawColumns) {
+		if (typeof col !== "string") return null;
+		columns.push(col);
+	}
+
+	const rows: string[][] = [];
+	for (const row of rawRows) {
+		if (!Array.isArray(row)) return null;
+		const cells: string[] = [];
+		for (const cell of row) {
+			const c = coerceCell(cell);
+			if (c === null) return null;
+			cells.push(c);
+		}
+		rows.push(cells);
+	}
+
+	return { columns, rows };
+}