feat(storage): Formatversionierung, Migrations-Backups und Fixture-Tests

tests/fixtures/future_v99_manifest.json

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+{
+  "format_version": 99,
+  "id": "future_notebook",
+  "title": "Zukunfts-Notizbuch",
+  "created_at": 1800000000,
+  "modified_at": 1800001000,
+  "current_page_index": 0,
+  "pages": [
+    { "id": 1, "background": "Grid5mm" }
+  ],
+  "page_ids": [1],
+  "next_stroke_id": 10,
+  "next_page_id": 2
+}

tests/fixtures/future_v99_page_1.json

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+{
+  "format_version": 99,
+  "id": 1,
+  "background": "Blank",
+  "strokes": []
+}

tests/fixtures/legacy_v0_manifest.json

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+{
+  "id": "legacy_notebook",
+  "title": "Legacy Notizbuch v0",
+  "created_at": 1700000000,
+  "modified_at": 1700001000,
+  "current_page_index": 0,
+  "page_ids": [1],
+  "next_stroke_id": 5,
+  "next_page_id": 2
+}

tests/fixtures/legacy_v0_page_1.json

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+{
+  "id": 1,
+  "background": "DotGrid",
+  "strokes": [
+    {
+      "id": 1,
+      "color": [0.1, 0.2, 0.3, 1.0],
+      "samples": [
+        { "x": 100.0, "y": 200.0, "p": 0.5, "t": 1.0 },
+        { "x": 110.0, "y": 210.0, "p": 0.6, "t": 1.1 }
+      ]
+    }
+  ]
+}

tests/fixtures/unknown_fields_page_1.json

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+{
+  "format_version": 1,
+  "id": 1,
+  "background": "Lined",
+  "strokes": [],
+  "unknown_experimental_feature": {
+    "nested_data": [1, 2, 3],
+    "author": "Antigravity"
+  },
+  "random_future_flag": true
+}