test(maps): maps_t1 cell-tile material round-trip + guards

Exercises set_cell_material / get_cell_material round-trip on the
write_test fixture, plus bounds rejection and unknown-layer
rejection. Confirms the new public API is wired and the guards
match the existing v3 write-API conventions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

init.lua

@@ -357,6 +357,37 @@ function M.run_tests(ctx)
     -- Cleanup
     os.remove(out_path); os.remove(out_path_2); os.remove(abs_rl_path)
 
+    -- =====================================================================
+    -- 0.5.7: Cell-Tile material API tests (maps_t1)
+    -- =====================================================================
+    -- Reload the write_test map for a clean slate.
+    maps.set_current("write_test")
+
+    -- maps_t1: round-trip set/get
+    maps.set_cell_material("surface", 1, 1, true)
+    engine.test.equals(maps.get_cell_material("surface", 1, 1), true,
+        "maps_t1: set_cell_material then get_cell_material round-trips")
+
+    maps.set_cell_material("surface", 1, 1, false)
+    engine.test.equals(maps.get_cell_material("surface", 1, 1), false,
+        "maps_t1: set_cell_material(false) clears the flag")
+
+    -- Out-of-bounds rejection
+    local ok_oob, err_oob = pcall(function()
+        maps.set_cell_material("surface", -1, 0, true)
+    end)
+    engine.test.assert(not ok_oob,
+        "maps_t1: set_cell_material rejects negative x")
+    engine.test.assert(err_oob and err_oob:find("bounds"),
+        "maps_t1: bounds-error mentions 'bounds'")
+
+    -- Unknown-layer rejection
+    local ok_lay = pcall(function()
+        maps.set_cell_material("bogus_layer", 0, 0, true)
+    end)
+    engine.test.assert(not ok_lay,
+        "maps_t1: set_cell_material rejects unknown layer")
+
     engine.exit(engine.test.failures())
 end