Fuzz NewManifestFromReader and cap the zstd decoder (closes #65)

FuzzNewManifestFromReader fails when the parser returns both or neither
of a manifest and an error, or allocates more than sixteen times its
input and the decompressed data it may read, plus room for the decoder's
window. make test runs the committed seed corpus; make fuzz fuzzes for
one minute, by hand only.

Parser bug: MaxDecompressedSize did not bound decompression. The zstd
decoder decoded a payload under 128 KiB in full, each frame up to its
own 64 GiB limit, before the LimitReader read any of it. It now decodes
synchronously, only what the LimitReader reads, with MaxDecompressedSize
as its limit. Regression seeds: a frame claiming 8 GiB, and two frames
each under the limit and together over it.

Model: opus-5-5

README.md

@@ -44,6 +44,9 @@ provide:
   such as `script/docker`
 - `script/test` — run the test suite (`go test`), regenerating the protobuf code
   first if it is stale
+- `script/fuzz` — fuzz the manifest parser for one minute; run by hand
+  (`make fuzz`), never by CI, while `script/test` runs its committed seed corpus
+  as ordinary tests
 - `script/lint` — run `golangci-lint` and verify `gofmt` cleanliness
 - `script/fmt` — format all code and docs (writes): `gofumpt`,
   `golangci-lint run --fix`, and `script/prettier --write`