feat(command-zone): add foundational type definitions

Add core type definitions and constants for the command zone feature:

- zone_names.h: Add command zone name constants (command, partner,
  companion, background) for protocol and lookup operations
- command_zone_types.h: Define CommandZoneType enum, ZoneVisibility enum,
  and StateChangeResult struct for state machine logic
- counter_ids.h: Define shared counter IDs and names for commander tax
  tracking (IDs 8-9 reserved to avoid conflicts with user counters)

These headers are intentionally minimal and dependency-free to enable
inclusion in both production code and isolated unit tests.

libcockatrice_network/libcockatrice/common/counter_ids.h

@@ -0,0 +1,29 @@
+#ifndef COUNTER_IDS_H
+#define COUNTER_IDS_H
+
+/**
+ * Shared counter IDs used by both client and server.
+ * These must match between server_player.cpp and player_event_handler.cpp.
+ *
+ * Reserved counter IDs for system counters:
+ *   IDs 0-7:  Standard player counters (life, mana colors, storm)
+ *   IDs 8-9:  Commander tax counters (reserved, do not use for custom counters)
+ *   IDs 10+:  Available for user-created counters
+ *
+ * The server's newCounterId() starts from the highest existing ID + 1,
+ * so these reserved IDs won't conflict as long as they're created first
+ * during setupZones().
+ */
+namespace CounterIds
+{
+constexpr int CommanderTax = 8;
+constexpr int PartnerTax = 9;
+} // namespace CounterIds
+
+namespace CounterNames
+{
+constexpr const char *CommanderTax = "commander_tax_counter";
+constexpr const char *PartnerTax = "partner_tax_counter";
+} // namespace CounterNames
+
+#endif // COUNTER_IDS_H