Bring the decoder over from the project it grew in

Eighteen commits of history, grafted onto the scaffold rather than over
it. It leaves behind everything that only meant something inside that
project: paths under one machine's mount, references to a transcoder
that is not here, and a benchmark defaulting to a tile nobody else has.

One fixture is real terrain and stays, because it is the only small
input that exercises the direct-nibble Huffman weight form. Its licence
requires attribution to travel with it, so the README's
acknowledgements and the corpus manifest both carry the Copernicus
notice now. Everything else committed here is synthetic or was written
by the zstd CLI, including the .hgtz container, whose name looks like a
tile and whose samples are a function of their position.

pubspec.yaml

@@ -0,0 +1,25 @@
+name: pure_zstd
+description: >-
+  A zstd decompressor written in plain Dart. Decodes RFC 8878 frames with no
+  native library and no platform channel, verified against libzstd.
+version: 0.1.0
+
+repository: https://github.com/tagadvance/pure-zstd
+issue_tracker: https://github.com/tagadvance/pure-zstd/issues
+
+# Remove this line to publish. A published version can be retracted but never
+# deleted, so it stays until the first release is deliberate.
+publish_to: none
+
+topics:
+  - zstd
+  - zstandard
+  - compression
+  - decompression
+
+environment:
+  sdk: ^3.8.0
+
+dev_dependencies:
+  lints: ^6.0.0
+  test: ^1.25.0