feat: gate a run of blocks to chosen output formats

gemtext/src/lib.rs

@@ -143,6 +143,9 @@ impl Writer {
             // Alignment and pagination have no expression here. Unwrapping keeps
             // the content; the presentation is simply not available.
             Block::Aligned { block, .. } => self.block(block),
+            // Gates are filtered out before rendering; keeping the content is
+            // the harmless reading if one ever arrives here.
+            Block::Gated { block, .. } => self.block(block),
             Block::CardBreak { .. } => {}
             // Raw markup would be shown literally, which is worse than omitting it.
             Block::Html(_) => {}