Merge sole-store context change into the next-side

Bring the immutable-state single-store change onto the Noise XX line.
The shared NodeContext is now the sole store; this merge applies the
next-only adaptations the master-side change couldn't carry:

- Remove node_profile from the Node struct (next-only field) so it lives
  solely in NodeContext; migrate its readers (tree/bloom/discovery/
  handshake negotiation) onto the node_profile() accessor. The two FMP
  negotiation sites hoist node_profile() into a local to avoid borrowing
  &self while a connection is mutably borrowed.
- leaf_only sets both is_leaf_only and node_profile via the context swap.
- Preserve the XX handshake/rekey structure (no identity-in-msg1; XX/XK
  initiator/responder constructors) while applying the startup_epoch()
  accessor migration.
- Tests: route profile selection through a make_test_node_with_profile
  helper (profile is immutable, set via Config flags) instead of poking
  the removed field.

cargo test --lib 1369/0; clippy -D warnings and release build clean.

src/node/tree.rs

@@ -37,7 +37,7 @@ impl Node {
         &mut self,
         peer_addr: &NodeAddr,
     ) -> Result<(), NodeError> {
-        if self.node_profile == crate::protocol::NodeProfile::Leaf {
+        if self.node_profile() == crate::protocol::NodeProfile::Leaf {
             return Ok(());
         }
         let now_ms = std::time::SystemTime::now()