Free drag & drop with fixed positions, empty cells and kept pages (0.2.31)

Tiles get an absolute grid slot instead of a packed order: a tile lands exactly
where it is dropped, dropping on another tile swaps the two, everything else
stays put, and empty cells and empty pages persist. The mockup previews the
result while dragging, a new page waits below the last one, "Pagina toevoegen"
keeps an empty page, empty cells accept the next tile from the picker, and the
mouse no longer selects text while dragging.

Storage and protocol stay version 1: `tiles[].slot` (0-47) and `pages` (1-8)
are additive, the layout message carries `slots` and `pages`. Firmware 0.2.26
places tiles on those slots (`place()` next to `pack()`); older firmware packs
in order as before and the editor says so.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/test_runtime_model.cpp

@@ -57,3 +57,47 @@ static void test_domains_and_packing() {
   assert(pack(m.tiles, 4, p) == 2 && p[3].page == 1 && p[3].slot == 0);
 }
 struct RunExtra { RunExtra() { test_domains_and_packing(); } } run_extra;
+// Explicit grid positions (0.2.26+): gaps stay empty, a wide card starts in the left column.
+static void test_explicit_slots() {
+  using namespace runtime_tiles;
+  Model m; bool changed = false, moved = false;
+  std::array<Placement, MAX_TILES> p;
+  // Validation: one slot per entity, inside eight pages, no duplicates.
+  assert(!m.set_layout({"light.a", "light.b"}, "Thuis", changed, {0}, moved));
+  assert(!m.set_layout({"light.a", "light.b"}, "Thuis", changed, {0, 48}, moved));
+  assert(!m.set_layout({"light.a", "light.b"}, "Thuis", changed, {3, 3}, moved));
+  assert(!m.configured);
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {0, 5, 7}, moved));
+  assert(changed && !moved && m.explicit_slots);
+  assert(place(m, p) == 2 && p[0].page == 0 && p[0].slot == 0 && p[1].slot == 5 && p[2].page == 1 && p[2].slot == 1);
+  // Same tiles elsewhere: states survive, only the pages re-place.
+  for (size_t i = 0; i < m.count; ++i) { m.tiles[i].received = true; m.tiles[i].state = "on"; }
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {2, 5, 7}, moved));
+  assert(!changed && moved && m.ready() && m.tiles[0].state == "on");
+  assert(place(m, p) == 2 && p[0].slot == 2);
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {2, 5, 7}, moved));
+  assert(!changed && !moved);
+  // A wide card on an odd slot snaps to its row start; the page count follows its footprint.
+  m.tiles[2].wide = true;
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {2, 5, 11}, moved));
+  assert(!changed && moved && place(m, p) == 2 && p[2].page == 1 && p[2].slot == 4);
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {2, 5, 12}, moved));
+  assert(place(m, p) == 3 && p[2].page == 2 && p[2].slot == 0);
+  // An empty page between two used pages stays a page; the last slot of page eight is allowed.
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {0, 1, 46}, moved));
+  assert(place(m, p) == 8 && p[2].page == 7 && p[2].slot == 4);
+  // Pages kept on purpose extend the count, never shrink it.
+  m.pages = 3; assert(place(m, p) == 8);
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {0, 1, 2}, moved));
+  assert(place(m, p) == 3); m.pages = 1; assert(place(m, p) == 1); m.pages = 200; assert(place(m, p) == MAX_PAGES); m.pages = 1;
+  // Without slots (older manager) the in-order packing returns.
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {}, moved));
+  assert(!changed && moved && !m.explicit_slots);
+  assert(place(m, p) == 1 && p[0].slot == 0 && p[1].slot == 1 && p[2].slot == 2);
+  assert(m.set_layout({"light.a", "light.b", "light.c"}, "Thuis", changed, {}, moved));
+  assert(!changed && !moved);
+  // Changing the tiles resets everything, including the positions.
+  assert(m.set_layout({"light.a", "light.z"}, "Thuis", changed, {1, 6}, moved));
+  assert(changed && !moved && !m.ready() && place(m, p) == 2 && p[0].slot == 1 && p[1].page == 1);
+}
+struct RunSlots { RunSlots() { test_explicit_slots(); } } run_slots;