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 a fixed multiple of
its input and the decompressed data it may read, plus room for the
decoder's window buffers. make test runs the seed corpus; make fuzz
fuzzes for one minute.

Parser bug: MaxDecompressedSize did not bound decompression; the zstd
decoder decoded a payload under 128 KiB in full before the LimitReader
read any of it. It now decodes only what the LimitReader reads and
refuses windows over the 8 MiB mfer writes with. Seeds: a frame claiming
8 GiB, two frames together over the limit, empty frames with growing
windows.

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`