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.

testing/ci-local.sh

@@ -233,6 +233,21 @@ run_build() {
         record "clippy" 1
         return 1
     fi
+
+    # Guard: the effectively-immutable state lives solely in NodeContext. The
+    # Node struct must not re-declare a bundled field (config/identity/
+    # startup_epoch/started_at/is_leaf_only/node_profile/max_*) — a shadow field
+    # would silently reopen dual-store divergence between the struct and the
+    # context. Checks the struct *declaration*, so it is wrap-insensitive.
+    info "node-context single-store guard"
+    if awk '/^pub struct Node \{/,/^\}/' src/node/mod.rs \
+        | grep -qE '^[[:space:]]+(config|identity|startup_epoch|started_at|is_leaf_only|node_profile|max_connections|max_peers|max_links):'; then
+        fail "Node struct re-declares a bundled immutable field; it must live solely in NodeContext"
+        record "node-context-guard" 1
+        return 1
+    else
+        record "node-context-guard" 0
+    fi
 }
 
 # ── Stage 2: Unit Tests ───────────────────────────────────────────────────