feat(fof-layer2): Tauri commands + frontend compose/comment routing

Three new Tauri commands surface Layer 2 to the frontend:

- create_post_with_fof_comments(content) → { postId }
  Builds the FoF gating block from the default persona's keyring and
  publishes a Mode 2 post (public body, FoF-gated comments).
- comment_on_fof_post(post_id_hex, body)
  Backwards-compat wrapper: comment_on_post now auto-detects FoF
  gating on the parent post and dispatches internally. Existing
  comment-send UI works unchanged.
- revoke_fof_commenter(post_id_hex, pub_x_index, reason_code)
  Wraps the Node helper. UI for per-comment revocation can hang off
  this without further Node changes.

Node::comment_on_post now branches on parent post fof_gating presence
— this is the central routing point so neither the frontend nor
existing comment-send code needs to know about FoF specifics. FoF
posts get FoF comments automatically; non-FoF posts get the legacy
path. Same Tauri command, same frontend handler.

Frontend:
- Compose: comment-perm-select gains a "Friends of Friends" option.
  When picked, compose dispatches to create_post_with_fof_comments
  instead of the standard create_post. Standard set_comment_policy
  diff fires after so receivers' four-check accept rule activates.
- Attachments + non-default-persona FoF posts are out-of-v1; compose
  reports + aborts rather than silently producing a non-FoF post.

Layer 2 backend + minimal-viable UI complete. Granular per-comment
revoke UI and access-grant UI deferred — Node + Tauri primitives
exist; the surfaces can be added without further crypto work.

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

crates/core/src/node.rs

@@ -4578,6 +4578,21 @@ impl Node {
         post_id: PostId,
         content: String,
     ) -> anyhow::Result<crate::types::InlineComment> {
+        // FoF Layer 2: if the post carries fof_gating, route through
+        // the FoF comment path so the comment is encrypted under
+        // CEK_comments + signed under priv_x. The CDN four-check accept
+        // rule on receivers will then validate the comment.
+        let is_fof_gated = {
+            let storage = self.storage.get().await;
+            storage.get_post(&post_id)
+                .ok()
+                .flatten()
+                .and_then(|p| p.fof_gating)
+                .is_some()
+        };
+        if is_fof_gated {
+            return self.comment_on_fof_post(post_id, content).await;
+        }
         self.comment_on_post_inner(post_id, content, None).await
     }
 
@@ -4817,6 +4832,59 @@ impl Node {
         Ok(())
     }
 
+    /// FoF Layer 2: author a comment on a FoF-gated post. Finds the
+    /// caller's unlock (any held V_x that matches one of the post's
+    /// slots), encrypts the body under CEK_comments, signs with the
+    /// per-V_x priv_x, attaches pub_x_index, stores locally, and
+    /// propagates via the standard engagement-diff path.
+    ///
+    /// Returns the constructed InlineComment. Errors if the post
+    /// isn't FoF-gated, or if no held V_x admits the caller.
+    pub async fn comment_on_fof_post(
+        &self,
+        post_id: PostId,
+        body: String,
+    ) -> anyhow::Result<crate::types::InlineComment> {
+        let (unlock, slot_binder_nonce, commenter_id, commenter_secret, post_author) = {
+            let storage = self.storage.get().await;
+            let post = storage.get_post(&post_id)?
+                .ok_or_else(|| anyhow::anyhow!("post not found"))?;
+            let gating = post.fof_gating.as_ref()
+                .ok_or_else(|| anyhow::anyhow!("post is not FoF-gated"))?;
+            let slot_binder_nonce = gating.slot_binder_nonce;
+            let unlock = crate::fof::find_unlock_for_post(&*storage, &post)?
+                .ok_or_else(|| anyhow::anyhow!("no held V_x unlocks this post — not in FoF set"))?;
+            let identity = storage.get_posting_identity(&unlock.persona_id)?
+                .ok_or_else(|| anyhow::anyhow!("unlocking persona not on device"))?;
+            (unlock, slot_binder_nonce, identity.node_id, identity.secret_seed, post.author)
+        };
+
+        let now = std::time::SystemTime::now()
+            .duration_since(std::time::UNIX_EPOCH)?
+            .as_millis() as u64;
+        let comment = crate::fof::build_fof_comment(
+            &post_id, &unlock, &slot_binder_nonce,
+            &commenter_id, &commenter_secret, &body, None, now,
+        )?;
+
+        // Store locally.
+        {
+            let storage = self.storage.get().await;
+            storage.store_comment(&comment)?;
+        }
+
+        // Propagate via engagement-diff path.
+        let diff = crate::protocol::BlobHeaderDiffPayload {
+            post_id,
+            author: post_author,
+            ops: vec![crate::types::BlobHeaderDiffOp::AddComment(comment.clone())],
+            timestamp_ms: now,
+        };
+        self.network.propagate_engagement_diff(&post_id, &diff, &post_author).await;
+
+        Ok(comment)
+    }
+
     /// FoF Layer 2: retroactively widen read+comment access on a
     /// FoF-gated post the caller authored by sealing a fresh wrap slot
     /// under the given V_x and appending it to the post's gating.