Limit how much fetch and check read for a manifest or a file (closes #168)

NewManifestFromReader reads at most one byte past MaxManifestSize, a new
constant of 258 MiB: the 256 MiB decompressed limit grown by zstd's worst
case of 1/256, plus 1 MiB for the signature, the signing key and the
other outer fields. It refuses a larger manifest. fetch, and check given
a URL, stop downloading a manifest one byte past the same size and report
it as too large; tests lower that size to keep their memory small. fetch
stops reading a file one byte past its listed size, so a longer body ends
in the size mismatch at once instead of filling the disk. docs/FORMAT.md
states the limit and gives the decompressed limit as 256 MiB, the size
the code uses.

Model: opus-5-5

internal/cli/manifest_loader.go

@@ -23,6 +23,10 @@ const manifestFetchTimeout = 30 * time.Second
 // its message.
 var errHTTPStatus = errors.New("HTTP")
 
+// errManifestTooLarge indicates a manifest download that passed
+// CLIApp.maxManifestSize.
+var errManifestTooLarge = errors.New("manifest exceeds maximum allowed size")
+
 // isHTTPURL returns true if the string starts with http:// or https://.
 func isHTTPURL(s string) bool {
 	return strings.HasPrefix(s, "http://") || strings.HasPrefix(s, "https://")