Phase 2b: control-post flow (delete/visibility) + remove BlobDeleteNotice

Replaces two persona-signed direct pushes with CDN-propagated control posts:
a single `VisibilityIntent::Control` post type whose content is a signed
`ControlOp` the receiver verifies and applies. Deletes and visibility updates
now flow through the same neighbor-manifest CDN path as regular content — no
direct recipient push needed for persona-signed ops.

Core pieces:
- `VisibilityIntent::Control` + `VisibilityIntent::Profile` variants.
- `ControlOp::DeletePost` / `ControlOp::UpdateVisibility` (JSON, ed25519-signed
  by the target post's author over op-specific byte strings).
- `crypto::{sign,verify}_control_{delete,visibility}` signing primitives.
- `control::build_delete_control_post` + `build_visibility_control_post`
  for authors to construct control posts.
- `control::receive_post` — unified incoming-post path used by all 6 receive
  sites. Verifies control signatures BEFORE storing, so bogus controls never
  enter storage and can't be re-propagated via neighbor-manifest diffs.
- `control::apply_control_post_if_applicable` — executes the op under the
  same storage guard as the insert.

Feed filter:
- Feeds (`get_feed`, `get_feed_page`, `list_posts_page`,
  `list_posts_reverse_chron`) now exclude `Control` and `Profile` posts so
  they propagate + tombstone without surfacing.
- Sync/export path (`list_posts_with_visibility`) keeps its own unfiltered
  query so control posts still propagate via CDN.

Wire protocol:
- `SyncPost` carries `intent: Option<VisibilityIntent>` so control posts
  arrive with their intent preserved.
- `BlobDeleteNotice` (0x95) removed — orphan blobs on remote holders evict
  naturally via LRU rather than via a persona-signed push. Code path,
  payload, sender, tests, and `delete_blob_with_cdn_notify` all gone.

Tests: control delete roundtrip (apply + tombstone) and wrong-author
rejection (not stored, not applied). 112/112 core tests pass.

crates/core/src/lib.rs

@@ -2,6 +2,7 @@ pub mod activity;
 pub mod blob;
 pub mod connection;
 pub mod content;
+pub mod control;
 pub mod crypto;
 pub mod http;
 pub mod export;