Phase 2d: profile posts signed by the posting identity

Display metadata (display_name, bio, avatar_cid) is no longer broadcast via
the ProfileUpdate direct push when the user edits their name. It travels as
a signed public post with VisibilityIntent::Profile, authored by the posting
identity, and propagates through the normal neighbor-manifest CDN path.

Core pieces:
- `types::ProfilePostContent` — JSON payload serialized into the post's
  content field. Ed25519 signature by the posting secret over length-prefixed
  display_name + bio + 32-byte avatar_cid (or zeros) + timestamp.
- `crypto::{sign,verify}_profile` with strict length prefixing to prevent
  extension attacks.
- New `profile` module: `build_profile_post`, `verify_profile_post`,
  `apply_profile_post_if_applicable`. Last-writer-wins by timestamp.
- `control::receive_post` now verifies Profile-intent posts upfront (same
  as Control) so bogus signatures never enter storage, and applies them
  after store so the `profiles` row updates atomically with the insert.

Node API:
- `Node::set_profile` rewritten: builds a signed Profile post, stores under
  intent=Profile, applies it locally (upserts the profiles row keyed by the
  posting identity), then propagates via `update_neighbor_manifests_as`.
  Stops calling `network.push_profile` — display changes no longer trigger
  a direct wire push.
- `Node::my_profile` / `has_profile` read by `default_posting_id` instead of
  `node_id`, matching where the row is written now.

ProfileUpdate (0x50) and push_profile stay for now — they still carry
routing-only data (anchors, recent_peers, preferred_peers) via
`sanitized_for_network_broadcast` and are used by `set_anchors` /
`set_public_visible`. Removing the routing fields would be a broader
cleanup; scoped out of this phase.

Tests: roundtrip verify+store, wrong-author rejection (not stored), and
older-timestamp ignored. 114 / 114 core tests pass.

crates/core/src/lib.rs

@@ -10,6 +10,7 @@ pub mod identity;
 pub mod import;
 pub mod network;
 pub mod node;
+pub mod profile;
 pub mod protocol;
 pub mod storage;
 pub mod stun;