feat(fof-layer4): supersedes_post_id field on Post

Adds an optional supersedes_post_id pointer to Post for the
"re-issue with narrower access" path. Author publishes a new post
that references an earlier one; receivers can render "this is a
re-issued version of an earlier post" + offer to view the original.

Covered by PostId = BLAKE3(Post) — receivers can verify it wasn't
forged by anyone but the author.

#[serde(default)] for back-compat; existing posts deserialize with
None. All existing Post construction sites bulk-updated to set the
field to None.

Storage::get_post returns None for the field today; a follow-up can
add a dedicated column if/when receivers need to render the
pointer. The author-side re-issue helper that creates posts with
this field set comes in the Tauri/UI slice — the wire shape is in
place now.

148 tests pass (no regressions).

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

crates/core/src/group_key_distribution.rs

@@ -62,6 +62,7 @@ pub fn build_distribution_post(
         attachments: vec![],
         timestamp_ms,
     fof_gating: None,
+    supersedes_post_id: None,
     };
     let post_id = compute_post_id(&post);
     let visibility = PostVisibility::Encrypted { recipients: wrapped_keys };
@@ -243,6 +244,7 @@ mod tests {
             attachments: vec![],
             timestamp_ms: 200,
         fof_gating: None,
+        supersedes_post_id: None,
         };
         let forged_vis = PostVisibility::Encrypted { recipients: wrapped };