Add a real test suite for the save system (previously had none)

Covers save.c (device discovery/orchestration), savedevice.c (the
generic device state machine and platform dispatch), and the Linux
platform backend (path building, availability checks, JSON read/write,
corrupt/missing-file handling), plus saveslot.c/savesettings.c JSON
round-trips. 88 tests across 5 files, all run against the real Linux
filesystem backend sandboxed to a temp $HOME (there's no mockable
platform layer - the hooks are compile-time macros, not function
pointers).

Deliberately locks in two existing behaviors rather than working around
them: saveSaveSettings() is a permanent no-op because nothing anywhere
ever sets SAVE.settingsDirty = true, and saveUpdate() unconditionally
rewrites settings back out the moment a device is found regardless of
that same dirty flag. Both are pre-existing, not introduced here.

Does not cover the SAVE_DEVICE_DATA_RAW blob codec (PSP/GameCube/Wii
only) or GameCube's 2-device fallback chain - neither compiles into the
Linux host test build.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

test/save/savetestfixture.h

@@ -0,0 +1,37 @@
+/**
+ * Copyright (c) 2026 Dominic Masters
+ *
+ * This software is released under the MIT License.
+ * https://opensource.org/licenses/MIT
+ */
+
+#pragma once
+#include "save/save.h"
+
+/**
+ * Points $HOME at a fresh, empty temporary directory and resets the global
+ * SAVE state via the real saveInit(). Save tests run against the real Linux
+ * filesystem backend (there is no fake/mockable platform layer to swap in -
+ * the platform hooks are compile-time macros, not function pointers), so
+ * this sandboxing is what keeps tests from touching the real user's
+ * $HOME/.dusk/saves. Matches cmocka's CMUnitTestSetup signature - pass
+ * directly to cmocka_unit_test_setup_teardown.
+ */
+int saveTestFixtureSetup(void **state);
+
+/**
+ * Recursively removes the temporary $HOME directory tree created by
+ * saveTestFixtureSetup and restores the real $HOME. Matches cmocka's
+ * CMUnitTestTeardown signature - pass directly to
+ * cmocka_unit_test_setup_teardown.
+ */
+int saveTestFixtureTeardown(void **state);
+
+/**
+ * Places a regular file where the ".dusk" directory needs to go, so a
+ * subsequent availability check's mkdirp() call fails with ENOTDIR instead
+ * of succeeding. Used to deterministically exercise the "device unavailable
+ * because its directory couldn't be created" path without relying on real
+ * filesystem permission tricks. Must be called after saveTestFixtureSetup.
+ */
+void saveTestFixtureBlockSaveDirectory(void);