feat: add deterministic simulation foundation

Replace implicit randomness with seeded per-NPC RandomNumberGenerator
streams so the same seed always produces the same scenario outcome.

- Add SimulationClock (RefCounted): explicit fixed-step clock that
  converts frame delta into simulation ticks while preserving sub-tick
  remainder; replaces raw _process tick_timer accumulation
- Add simulation_seed export (int=1337) to SimulationManager; drive
  per-NPC RNG streams via seed + npc_id + stream_id derivation
  (stream 0=init, stream 1=wander)
- Add get_wander_offset(npc_id) to SimulationManager so wander targets
  are deterministic per seed; WorldViewManager calls it instead of
  local randf_range()
- Add get_state_snapshot() and get_state_checksum() to SimulationManager
  for headless verification and future save/load
- Convert SimNPC/SimVillage const DEBUG_LOGS to instance var debug_logs
  so headless tests can silence output without recompilation
- Pass RandomNumberGenerator through SimNPC._init() instead of using
  global randf_range() for hunger/energy/position/scoring noise
- Remove obsolete farm_zone and forest_zone exports from
  WorldViewManager and their marker/tree scene children from main.tscn;
  NPC gather tasks use ResourceNode exclusively
- Rename TaskZone container to ActivityMarkers in main.tscn and update
  all scene paths (player, WVM, tests) to match
- Add headless deterministic_simulation_test.gd: verifies clock
  accuracy, same-seed equality, different-seed divergence, and
  24-tick scenario checksum without loading main.tscn

tests/flat_map_baseline_test.gd

@@ -28,10 +28,10 @@ func _run() -> void:
 	]
 	var destinations: Array[Vector3] = []
 	for marker_path in [
-		"TaskZone/FoodZone",
-		"TaskZone/GuardZone",
-		"TaskZone/StudyZone",
-		"TaskZone/RestZone"
+		"ActivityMarkers/FoodZone",
+		"ActivityMarkers/GuardZone",
+		"ActivityMarkers/StudyZone",
+		"ActivityMarkers/RestZone"
 	]:
 		destinations.append(main_scene.get_node(marker_path).global_position)