feat: tosc sections page for song build

Add SECTIONS (page 4): 8 slot launch+save buttons, PREV/NEXT nav,
/scene/state receive on the GROUP so sections.lua colors slots
(green=current, blue=filled, dark=empty). Update pager to 4 pages,
add 4 new tests (26 pass), rebuild dist/av-live-control.tosc.

touchosc/gen/lua/sections.lua

@@ -0,0 +1,23 @@
+-- SECTIONS GROUP: color the 8 launch slot buttons from /scene/state.
+-- API verified on GrosMac at fidelity gate.
+-- /scene/state <cur:int> <fill0:int> ... <fill7:int>
+-- cur=-1 means no active section; fillN=1 means slot N is saved.
+function onReceiveOSC(message, connections)
+  local path = message[1]
+  if path ~= "/scene/state" then return end
+  local args = message[2]
+  local cur = math.floor((args[1] and tonumber(args[1].value)) or -1)
+  for i = 0, 7 do
+    local fill = math.floor((args[i + 2] and tonumber(args[i + 2].value)) or 0)
+    local slot = self:findByName("slot" .. i, true)
+    if slot then
+      if i == cur then
+        slot.color = Color(0.27, 0.8, 0.4, 1)        -- current (green)
+      elseif fill == 1 then
+        slot.color = Color(0.2, 0.4, 0.8, 1)          -- filled (blue)
+      else
+        slot.color = Color(0.15, 0.15, 0.15, 1)       -- empty (dark)
+      end
+    end
+  end
+end