Test the guards that had no test

A sweep found no defect in the decoder and several in its tests. Three
of the five fixes could be reverted with all 103 tests still green.

The content size against expectedSize check is the worst of those,
because the .hgtz reader was simplified to rest on it: deleting
it left everything passing. Eleven cases cover it now. The RLE block
ceiling and the requirement that a frame be the whole input had none
either, so there are fixtures for a 13-byte frame that returns 2 MB, for
the same shape at exactly the maximum, for four bytes of junk after a
frame, and for a trailing skippable frame.

Two of the new fixtures close coverage gaps rather than guards. No legal
frame in the suite decoded a one-stream Huffman literals section, since
the only one present had to be refused. And the only test of hashing
each block as it lands rode on the zstd CLI being installed, so a
multi-block frame carrying a checksum is committed now.

Also replaces an assertion that constrained nothing. "Fills the buffer
exactly" checked that the count was over 256 and under 1024, which
passes with the guard at two, at three, or absent. The stream encodes
257 symbols, a buffer of 257 takes it and one of 256 does not.

And two comments that were wrong. The weight buffer is one larger than
the maximum on purpose, and shrinking it to 255 breaks 124 tests. A
caller passing expectedSize gets the exact check in addition to the
ceiling, not instead of it.

test/fse_interleaved_test.dart

@@ -38,18 +38,26 @@ void main() {
       );
     });
 
-    test('fills the buffer exactly when there is room for the last pass', () {
-      final table = FseTable(6);
-      readFseTable(table, description, 0, description.length, 255);
+    test(
+      'writes exactly what the stream encodes, and knows where that ends',
+      () {
+        // The old version of this asserted only that the count was over 256
+        // and under 1024, which passes with the guard at `at + 2`, at
+        // `at + 3`, or removed entirely. This stream encodes 257 symbols, and
+        // the boundary is the whole point: a buffer of exactly 257 takes it
+        // and one of 256 does not.
+        int run(int limit) {
+          final table = FseTable(6);
+          readFseTable(table, description, 0, description.length, 255);
+          final bits = ReverseBitReader()..reset(stream, 0, stream.length);
 
-      // Same stream, a buffer with slack. The count it returns must be the
-      // symbols it actually wrote, so the last pass cannot have been longer
-      // than the room the check left.
-      final bits = ReverseBitReader()..reset(stream, 0, stream.length);
-      final room = Uint8List(1024);
-      final written = fseDecodeInterleaved(table, bits, room, 0, room.length);
-      expect(written, greaterThan(256));
-      expect(written, lessThanOrEqualTo(room.length));
-    });
+          return fseDecodeInterleaved(table, bits, Uint8List(1024), 0, limit);
+        }
+
+        expect(run(1024), 257);
+        expect(run(257), 257);
+        expect(() => run(256), throwsA(isA<ZstdException>()));
+      },
+    );
   });
 }