diff --git a/src/state.lua b/src/state.lua
new file mode 100644
index 0000000..cd3227f
--- /dev/null
+++ b/src/state.lua
@@ -0,0 +1,96 @@
+-- src/state.lua
+-- Editor state singleton + accessors.
+
+local M = {}
+
+-- =====================================================================
+-- State (module-singleton)
+-- =====================================================================
+local state = {
+    map_id        = nil,                -- set by init.lua after maps.load()
+    map_path      = "maps/work.map.json",
+    active_layer  = "surface",          -- one of VALID_LAYER_NAMES
+    active_atlas  = 0,                  -- atlas_index 0..N-1
+    active_tile   = 1,                  -- tile_id
+    active_rot    = 0,                  -- 0..3
+    roof_mode     = false,              -- if true, paint targets roof
+    layer_visible = {
+        foundation=true, subsurface=true, surface=true, topsurface=true,
+        lower_wall=true, wall=true, upper_wall=true, canopy=true,
+    },
+    dirty         = false,
+    picker_open   = false,              -- atlas+tile picker expanded?
+    mouse_cell    = nil,                -- {x, y} or nil if mouse off-map
+}
+
+-- =====================================================================
+-- Setters / getters (encapsulate state for testability)
+-- =====================================================================
+
+function M.set_map(map_id, path)
+    state.map_id = map_id
+    state.map_path = path
+end
+
+function M.get_map_id()     return state.map_id end
+function M.get_map_path()   return state.map_path end
+
+function M.set_active_layer(name)  state.active_layer = name end
+function M.get_active_layer()      return state.active_layer end
+
+function M.set_active_atlas(idx)   state.active_atlas = idx end
+function M.get_active_atlas()      return state.active_atlas end
+
+function M.set_active_tile(id)     state.active_tile = id end
+function M.get_active_tile()       return state.active_tile end
+
+function M.cycle_rotation()
+    state.active_rot = (state.active_rot + 1) % 4
+end
+function M.get_active_rot()        return state.active_rot end
+
+function M.toggle_roof_mode()
+    state.roof_mode = not state.roof_mode
+end
+function M.is_roof_mode()          return state.roof_mode end
+
+function M.toggle_layer_visible(name)
+    state.layer_visible[name] = not state.layer_visible[name]
+end
+function M.is_layer_visible(name)  return state.layer_visible[name] ~= false end
+
+function M.mark_dirty()            state.dirty = true end
+function M.mark_clean()            state.dirty = false end
+function M.is_dirty()              return state.dirty end
+
+function M.toggle_picker_open()
+    state.picker_open = not state.picker_open
+end
+function M.is_picker_open()        return state.picker_open end
+
+function M.set_mouse_cell(x, y)
+    if x == nil then
+        state.mouse_cell = nil
+    else
+        state.mouse_cell = { x = x, y = y }
+    end
+end
+function M.get_mouse_cell()        return state.mouse_cell end
+
+-- Used by tests to reset between asserts
+function M.reset()
+    state.map_id = nil
+    state.active_layer = "surface"
+    state.active_atlas = 0
+    state.active_tile = 1
+    state.active_rot = 0
+    state.roof_mode = false
+    state.dirty = false
+    state.picker_open = false
+    state.mouse_cell = nil
+    for name in pairs(state.layer_visible) do
+        state.layer_visible[name] = true
+    end
+end
+
+return M