add fragment_file_spec.md

doc/specs/fragment_file_spec.md

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+# Fragment file (.vox)
+
+Current version is 1.
+
+Fragment file is [vcbjson file](binary_json_spec.md).
+
+JSON schema representation format in [Orderly](https://orderly-json.org/docs/) language:
+
+```
+object {
+    integer version {1,};
+    array {integer;} {1,} size;
+    array {string;} block-names;
+    array {integer;} {0,65535} voxels;
+}
+```
+
+Where:
+- **version** is fragment file format version.
+- **size** is fragment size (three positive integers).
+- **block-names** is array of full block names: `pack_id:block_name`.
+- **voxels**: voxels data array: *index, state, index, state...*
+  where:
+  - index is based on block-names array.
+  - state - see [block state](region_voxels_chunk_spec.md#block-state).
+
+**block-names** must have `core:air` as the first element.