Add runtime-loaded cutscene files, convert initial/main menu to use them

New ASSET_LOADER_TYPE_CUTSCENE (assetcutsceneloader.c) reads a versioned
binary .cts format decoded into a heap-allocated cutsceneitem_t array + a
string/data pool, both sized to the file's actual content rather than a
fixed capacity, so the shared assetloaderoutput_t union doesn't bloat for
every asset slot regardless of type. Authoring pipeline mirrors the chunk
asset pattern: assetsraw/cutscenes/*.jsonc (JSON plus // and /* */
comments) -> tools/asset/cutscene -> assets/cutscenes/*.cts.

cutsceneSystemSetOnComplete() lets the caller arm a native callback that
fires when a cutscene finishes normally, so a file (which can't store a
function pointer) can end plainly and still hand off to native code -
cutsceneRestart() preserves it across a retry loop rather than clearing it,
since a restart is the same logical run trying again.

The initial and main-menu start-game cutscenes are now loaded from files
instead of compiled in via the CUTSCENE(...) macro.

Fixed a real bug found while converting these: the sync loader read the
file's total size from assetfile_t.size to locate the trailing pool
region, but assetFileDispose() (called at the end of the async phase)
zeroes that whole struct first, so the size was always 0 and the pool
offset computation underflowed into an out-of-bounds read - intermittent
depending on heap layout. Fixed by saving the size before disposal; also
fixed the read-completeness assert being checked after that same zeroing
(a no-op 0 == 0 check).

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

assetsraw/cutscenes/initial.jsonc

@@ -0,0 +1,69 @@
+{
+  "items": [
+    // Boot check: make sure a save device is available before handing off
+    // to the main menu.
+    {
+      "type": "MODAL",
+      "title": "initial.checking_save.title",
+      "message": "initial.checking_save.message"
+    },
+    {
+      "type": "WAIT",
+      "seconds": 0.2
+    },
+    {
+      "type": "SAVE_DEVICE_CHECK",
+      "successMarker": "CONTINUE",
+      "failureMarker": "NO_DEVICE"
+    },
+
+    // No save device found - offer to retry or continue without saving.
+    {
+      "type": "MARKER",
+      "name": "NO_DEVICE"
+    },
+    {
+      "type": "MODAL_CLOSE"
+    },
+    {
+      "type": "MODAL_OPTIONS_MARKERS",
+      "title": "initial.no_device.title",
+      "message": "initial.no_device.message",
+      "options": [
+        {
+          "text": "initial.no_device.retry",
+          "marker": "RETRY"
+        },
+        {
+          "text": "initial.no_device.continue",
+          "marker": "CONTINUE"
+        }
+      ]
+    },
+
+    {
+      "type": "MARKER",
+      "name": "RETRY"
+    },
+    {
+      "type": "MODAL_CLOSE"
+    },
+    {
+      "type": "RESTART"
+    },
+
+    // Save device found (or continuing without one) - hand off to the
+    // main menu scene.
+    {
+      "type": "MARKER",
+      "name": "CONTINUE"
+    },
+    {
+      "type": "MODAL_CLOSE"
+    },
+    {
+      "type": "SCENE",
+      "sceneType": "MAIN_MENU"
+    }
+  ]
+}

assetsraw/cutscenes/main_menu_start_game.jsonc

@@ -0,0 +1,92 @@
+{
+  "items": [
+    {
+      "type": "MODAL",
+      "title": "main_menu.checking_save.title",
+      "message": "main_menu.checking_save.message"
+    },
+    {
+      "type": "WAIT",
+      "seconds": 0.2
+    },
+    {
+      "type": "SAVE_DEVICE_CHECK",
+      "successMarker": "CONTINUE",
+      "failureMarker": "NO_DEVICE"
+    },
+
+    // No save device found - offer to retry or continue without saving.
+    {
+      "type": "MARKER",
+      "name": "NO_DEVICE"
+    },
+    {
+      "type": "MODAL_CLOSE"
+    },
+    {
+      "type": "MODAL_OPTIONS_MARKERS",
+      "title": "main_menu.no_device.title",
+      "message": "main_menu.no_device.message",
+      "options": [
+        {
+          "text": "main_menu.no_device.retry",
+          "marker": "RETRY"
+        },
+        {
+          "text": "main_menu.no_device.continue",
+          "marker": "CONTINUE"
+        }
+      ]
+    },
+
+    // Retry
+    {
+      "type": "MARKER",
+      "name": "RETRY"
+    },
+    {
+      "type": "MODAL_CLOSE"
+    },
+    {
+      "type": "RESTART"
+    },
+
+    // Save device found - attempt to load all save slots.
+    {
+      "type": "MARKER",
+      "name": "CONTINUE"
+    },
+    {
+      "type": "MODAL_CLOSE"
+    },
+    {
+      "type": "SAVE_LOAD_ALL_SLOTS",
+      "successMarker": "LOADED",
+      "failureMarker": "LOAD_ERROR"
+    },
+
+    // Save data failed to load (e.g. corrupt/unreadable) - only option is
+    // to retry, no "continue without saving" here since we already know a
+    // device is present.
+    {
+      "type": "MARKER",
+      "name": "LOAD_ERROR"
+    },
+    {
+      "type": "MODAL_OPTIONS_MARKERS",
+      "title": "main_menu.save_load_error.title",
+      "message": "main_menu.save_load_error.message",
+      "options": [
+        {
+          "text": "main_menu.save_load_error.retry",
+          "marker": "RETRY"
+        }
+      ]
+    },
+
+    {
+      "type": "MARKER",
+      "name": "LOADED"
+    }
+  ]
+}