feat(fof-layer2): wire FoF gating into post-create path

Threads optional fof_gating through create_post_inner so a published
post can carry the author-signed gating snapshot at publish time
(covered by PostId = BLAKE3(Post)).

New public entry point:

  Node::create_post_with_fof_comments(content, attachments)
      -> (PostId, Post, PostVisibility, cek: [u8; 32])

Builds the FoFCommentGating block via fof::build_fof_comment_gating
from the default persona's keyring (own V_me + every received V_x),
then calls create_post_inner with VisibilityIntent::Public (Mode 2
keeps body public). Returns the per-post CEK to the caller for local
caching (decrypting one's own comments later).

Existing create_post / create_post_as / create_post_with_visibility
threads `None` through the new arg — back-compat for non-FoF posts.

CommentPolicy::FriendsOfFriends is NOT published as a SetPolicy diff
in this commit; the post's `fof_gating` field is itself the signal
that the post supports FoF commenting. The four-check CDN verify gate
(next commit) reads fof_gating directly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/core/src/node.rs

@@ -1014,6 +1014,7 @@ impl Node {
             content,
             intent,
             attachment_data,
+            None,
         ).await
     }
 
@@ -1038,9 +1039,43 @@ impl Node {
             content,
             intent,
             attachment_data,
+            None,
         ).await
     }
 
+    /// FoF Layer 2: create a Mode 2 post (public body, FoF-gated
+    /// comments). Intent is Public; the FoF gating block is built
+    /// from the default persona's keyring and embedded in
+    /// `Post.fof_gating`. The author retains the per-post CEK locally
+    /// for decrypting their own comments later.
+    ///
+    /// Returns `(post_id, post, visibility, cek)`. `visibility` is
+    /// always Public for Mode 2.
+    pub async fn create_post_with_fof_comments(
+        &self,
+        content: String,
+        attachment_data: Vec<(Vec<u8>, String)>,
+    ) -> anyhow::Result<(PostId, Post, PostVisibility, [u8; 32])> {
+        // Build the gating block from the default persona's keyring.
+        let built = {
+            let storage = self.storage.get().await;
+            crate::fof::build_fof_comment_gating(&*storage, &self.default_posting_id)?
+                .ok_or_else(|| anyhow::anyhow!(
+                    "default persona has no V_me; rotate or recreate before FoF posts"
+                ))?
+        };
+        let cek = built.cek;
+        let (post_id, post, visibility) = self.create_post_inner(
+            &self.default_posting_id,
+            &self.default_posting_secret,
+            content,
+            VisibilityIntent::Public,
+            attachment_data,
+            Some(built.gating),
+        ).await?;
+        Ok((post_id, post, visibility, cek))
+    }
+
     async fn create_post_inner(
         &self,
         posting_id: &NodeId,
@@ -1048,6 +1083,7 @@ impl Node {
         content: String,
         intent: VisibilityIntent,
         attachment_data: Vec<(Vec<u8>, String)>,
+        fof_gating: Option<crate::types::FoFCommentGating>,
     ) -> anyhow::Result<(PostId, Post, PostVisibility)> {
         // Validate attachments
         if attachment_data.len() > 4 {
@@ -1163,7 +1199,7 @@ impl Node {
             content: final_content,
             attachments,
             timestamp_ms: now,
-        fof_gating: None,
+            fof_gating,
         };
 
         let post_id = compute_post_id(&post);