Fix StubReader.ReadByte() discarding valid byte on EOF

[hdrover]

StubReader.ReadByte() ignores the n return value from Read().

When Read() returns (n=1, err=io.EOF), which is valid per the io.Reader contract, ReadByte() returns (byte, EOF) instead of (byte, nil). Callers see err != nil and discard the successfully read byte.

This causes readString() in libbox/profile_import.go to fail with unexpected EOF when decoding .bpf profile files if the gzip stream delivers the last byte together with EOF.

Fix: replace r.Read() with io.ReadFull(), which returns nil when the requested number of bytes has been fully read, even if the underlying Read() returned EOF alongside the data.

This became a visible regression in sing-box v1.13.0 after commit SagerNet/sing-box@1af14a0 ("Remove varbin usages"), which changed string reading from io.ReadFull() (tolerant to data+EOF) to a byte-by-byte ReadByte() loop, exposing this bug. In v1.12.x profile import worked correctly.

Screenshot: sing-box Android app failing to import a valid .bpf file generated with standard gzip (without sync flush).