Multi-network foundation: netmgr, derived identities, additive IPC protocol

YAW/2 peer wire protocol is unchanged. Changes are local only.

internal/crypto:
- DeriveForNetwork(master, networkHash) — HKDF-SHA256 from master seed + network hash;
  same master + same network always produces the same Ed25519 keypair (stable peer ID)

internal/proto:
- NetworkInfo type (network_id, network_name, local_peer)
- NetworkID field on IpcMessage (optional; commands default to first network when absent)
- Networks []NetworkInfo on state_snapshot (additive alongside existing local_peer)
- EvtNetworkJoined / EvtNetworkLeft events

internal/netmgr (new):
- Manager holds N independent Network contexts (derived identity, mesh, store, anchor)
- Join(name) creates context, derives identity, opens per-network DB, starts anchor client
- Leave(id) / LeaveAll() cancel contexts and close stores
- Resolve(netID) returns named network, or Default() when netID is empty (backward compat)
- Fan-out: Manager.Subscribe() receives tagged events from all networks
- Network IDs are the first 8 hex chars of SHA-256("yaw2-net:"+name) — stable and short

internal/ipc:
- Run(mgr, port) replaces Run(m *mesh.Mesh, port, anchorURL, joinFn)
- Commands without network_id route to mgr.Default() (backward compat)
- state_snapshot includes Networks array; local_peer/connected_peers still populated from first network
- generate_invite, get_file_list, send_message all respect network_id routing

cmd/daemon:
- Creates netmgr.Manager instead of mesh.Mesh directly
- --join and --share-dir pass through Config
- Auto-join via mgr.Join() before IPC starts

test-network.sh:
- Fix peer_name bash bug: $() with && inside triggers set -e; use if/elif/else instead

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test-network.sh

@@ -319,7 +319,9 @@ echo -e "file listing"
 echo -e "${DIM}────────────────────────────────────────────────────────${RESET}"
 
 for peer_ipc in "$ALICE_IPC" "$BOB_IPC" "$CHARLIE_IPC"; do
-    peer_name=$([ "$peer_ipc" = "$ALICE_IPC" ] && echo "alice" || ([ "$peer_ipc" = "$BOB_IPC" ] && echo "bob" || echo "charlie"))
+    if [ "$peer_ipc" = "$ALICE_IPC" ]; then peer_name="alice"
+    elif [ "$peer_ipc" = "$BOB_IPC" ]; then peer_name="bob"
+    else peer_name="charlie"; fi
     result=$(echo '{"type":"get_file_list"}' \
         | timeout 2 nc 127.0.0.1 "$peer_ipc" 2>/dev/null \
         | grep '"type":"file_list"' | head -1)