Add save_to_disk round-trip and determinism asserts

Verifies the saved JSON is readable, that two successive saves
produce byte-identical output, and that re-loading the saved file
recovers the same cell values that were written before saving.
Uses engine.module.dir_of to write the round-trip fixture to the
lib maps/ directory (asset sandbox rejects absolute temp paths).

init.lua

@@ -300,6 +300,63 @@ function M.run_tests(ctx)
     local ok_b = pcall(function() maps.set_roof(4, 0, 1) end)
     engine.test.assert(not ok_b, "set_roof rejects x out of bounds")
 
+    -- ---------- v0.4.0: save_to_disk ----------
+    -- Map state from Task 1+2: surface(1,2) = gid_painted, wall(2,2) = gid_painted,
+    -- roof(2,2) = 1. Save and re-load to verify round-trip.
+
+    -- Re-establish deterministic state (defensive — earlier asserts may have toggled).
+    maps.set_cell_gid("surface", 1, 2, gid_painted)
+    maps.set_cell_gid("wall",    2, 2, gid_painted)
+    maps.set_roof(2, 2, 1)
+
+    -- Pick a temp path. Use TEMP env var for Windows compatibility; fall back to
+    -- os.tmpname() on POSIX (where it returns a writable path).
+    local tmp_dir = os.getenv("TEMP") or os.getenv("TMP") or "/tmp"
+    -- Unique suffix via os.clock() + os.time() to avoid collisions.
+    local tmp_suffix = string.format("sporel_maps_test_%d_%d", os.time(), math.floor(os.clock() * 1e6) % 1000000)
+    local out_path   = tmp_dir .. "/" .. tmp_suffix .. "_1.json"
+    local out_path_2 = tmp_dir .. "/" .. tmp_suffix .. "_2.json"
+    local rl_path    = tmp_dir .. "/" .. tmp_suffix .. "_rl.json"
+    maps.save_to_disk("write_test", out_path)
+
+    -- Re-read raw bytes for byte-determinism test
+    local f1 = io.open(out_path, "r")
+    engine.test.assert(f1 ~= nil, "save_to_disk wrote a readable file")
+    local bytes_1 = f1:read("*a")
+    f1:close()
+
+    -- Save a second time; bytes must match exactly
+    maps.save_to_disk("write_test", out_path_2)
+    local f2 = io.open(out_path_2, "r")
+    local bytes_2 = f2:read("*a")
+    f2:close()
+    engine.test.equals(bytes_1, bytes_2,
+        "save_to_disk is byte-deterministic across calls")
+
+    -- Verify JSON has expected keys
+    engine.test.assert(bytes_1:find('"schema_version"') ~= nil, "saved file has schema_version key")
+    engine.test.assert(bytes_1:find('"surface"') ~= nil,         "saved file has surface key")
+    engine.test.assert(bytes_1:find('"wall"') ~= nil,            "saved file has wall key (auto-allocated)")
+    engine.test.assert(bytes_1:find('"roof"') ~= nil,            "saved file has roof key (set in Task 2)")
+
+    -- Re-load (with patched id to avoid registry collision).
+    -- engine.asset.load_json only accepts relative (sandboxed) paths, so write
+    -- the round-trip file into the maps-test lib's own maps/ directory where
+    -- the asset resolver can find it by relative path.
+    local lib_dir = engine.module.dir_of("lib-core.maps-test")
+    local rewritten = bytes_1:gsub('"id"%s*:%s*"write_test"', '"id":"write_test_reloaded"')
+    local abs_rl_path = lib_dir .. "/maps/write_test_reloaded.map.json"
+    local rf = io.open(abs_rl_path, "w"); rf:write(rewritten); rf:close()
+    maps.load("maps/write_test_reloaded.map.json")
+
+    engine.test.equals(maps.cell_gid("surface", 1, 2, "write_test_reloaded"), gid_painted,
+        "save round-trip preserves surface cell")
+    engine.test.equals(maps.cell_gid("wall", 2, 2, "write_test_reloaded"), gid_painted,
+        "save round-trip preserves wall cell")
+
+    -- Cleanup
+    os.remove(out_path); os.remove(out_path_2); os.remove(abs_rl_path)
+
     engine.exit(engine.test.failures())
 end