Decode frames that declare no content size

Streaming encoders leave Frame_Content_Size out unless told the size up front,
and their readers often can't know it either: every zstd frame in an Anki
package (its collection, its media list, each media file) is written that way.
Such a frame needed the exact size passed as expectedSize, and anything else
was refused.

Now it gets the most its blocks can hold, Block_Maximum_Size (128 KiB) each,
found by walking the block headers alone, and the output is trimmed to what
the blocks held. The output is still allocated once. expectedSize keeps its
exact check when given, and maxOutputSize bounds the allocation as before.

Checked against libzstd (python-zstandard) on Anki 26.09.3's exports: a small
package's four frames and a 30 MB collection decode byte for byte the same.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -32,8 +32,11 @@ for (final frame in frames) {
 }
 ```
 
-A frame that declares no content size needs `expectedSize`, because the
-output is allocated whole rather than grown.
+The output is allocated whole rather than grown. A frame that declares no
+content size, as streaming encoders write them, gets the most its blocks can
+hold (128 KiB each, found from their headers alone) and is trimmed to what
+they held; `expectedSize` turns that into an exact check when the caller
+knows the size.
 
 ## What it implements