[BUG] `nanovdb::io::readGrid` requires seekable istream but fails silently

[yzx9]

Environment

Operating System: Linux Ubuntu 24.04
Version / Commit SHA: VDB 12.0.1
Other: Not related

Describe the bug

nanovdb::io::readGrid(stream) assumes the input stream supports seeking. If a non-seekable std::istream is passed (e.g., a custom std::streambuf that does not implement seekoff), the function silently fails or hangs, without reporting any error.

To Reproduce

Steps to reproduce:

Define a minimal non-seekable std::streambuf:

struct NonSeekableBuf : std::streambuf {
    std::vector<char> buffer;

    NonSeekableBuf(const std::string& filename) {
        std::ifstream file(filename, std::ios::binary);
        if (!file) throw std::runtime_error("Failed to open file");

        file.seekg(0, std::ios::end);
        std::streamsize size = file.tellg();
        file.seekg(0, std::ios::beg);

        buffer.resize(size);
        file.read(buffer.data(), size);

        setg(buffer.data(), buffer.data(), buffer.data() + size);
    }

    // disable seek
    std::streampos seekoff(std::streamoff, std::ios_base::seekdir, std::ios_base::openmode) override {
        return std::streampos(std::streamoff(-1));
    }

    std::streampos seekpos(std::streampos, std::ios_base::openmode) override {
        return std::streampos(std::streamoff(-1));
    }
};

Use it with nanovdb::io::readGrid:

NonSeekableBuf buf("volume.nvdb");
std::istream in(&buf);

auto handle = nanovdb::io::readGrid<nanovdb::HostBuffer>(in);

Observe that:

• No grid is returned
• No error is thrown
• May hang in certain configurations

Expected behavior

The function should:

• Fail early and clearly if the stream is not seekable
• Optionally support non-seekable input by reading sequentially (if feasible)

Additional context

(Add any other context about the problem here.)