decode rejects header-only stream (valid empty Snappy frame)

[GrapeBaBa]

Bug

snappyframesz.decode rejects a stream that contains only the 10-byte stream-identifier chunk (no data chunks), even though the Snappy framing spec makes this a valid representation of an empty payload.

Repro

const std = @import("std");
const frames = @import("snappyframesz");

pub fn main() !void {
    const allocator = std.heap.page_allocator;
    // Stream identifier chunk only — no data chunks.
    // ff 06 00 00 sNaPpY  (10 bytes)
    const header_only = "\xff\x06\x00\x00sNaPpY";

    // Sanity: encoding the empty payload through the same lib produces
    // additional chunks because finish() forces an empty data chunk
    // when nothing was written.
    const encoded_via_lib = try frames.encode(allocator, "");
    defer allocator.free(encoded_via_lib);
    std.debug.print("lib encode(\"\").len = {d}\n", .{encoded_via_lib.len});

    // The header-only form is what every other Snappy implementation
    // accepts (Go `snappy.NewReader`, Rust `snap`). snappyframesz
    // returns FrameError.NotFramed instead of an empty slice.
    const decoded = try frames.decode(allocator, header_only);
    defer allocator.free(decoded);
    std.debug.print("decode(header_only).len = {d}\n", .{decoded.len});
}

Actual: error: NotFramed
Expected: [] (empty slice)

Root cause

src/frames.zig:276 (current master):

```zig
if (!saw_data_chunk) return FrameError.NotFramed;
```

The function exits successfully only after at least one data chunk has been seen. A stream that contains only the stream identifier — which is a perfectly valid empty Snappy frame — falls through this branch and returns NotFramed. The outer decode then falls back to snappyz.decode on the magic-header bytes, which also fails.

Cross-impl behaviour

Same 10-byte input `ff 06 00 00 sNaPpY`:

Impl	Result
Go `snappy.NewReader`	OK, `[]byte{}`
Rust `snap::read::FrameDecoder`	OK, `b""`
Python `cramjam`	OK
snappyframesz (this lib)	`FrameError.NotFramed`

This bites cross-client interop testing — leanSpec emits exactly this 10-byte stream as the canonical empty-input fixture for `snappy_frame`, and the zeam spec-test runner currently has to skip that one fixture explicitly.

Suggested fix

Drop the `saw_data_chunk` guard, or make it conditional on having seen neither a stream identifier nor a data chunk:

```zig
if (!saw_stream_identifier and !saw_data_chunk) return FrameError.NotFramed;
```

A stream identifier alone is a complete (empty) frame; the function should return an empty slice in that case.

Discovered via

zeam PR #715 — implementing the leanSpec `networking_codec` spec-test runner. The empty-input snappy_frame fixture (`leanSpec/fixtures/consensus/networking_codec/devnet/networking/test_snappy_codec/test_snappy_frame_empty.json`, expectedFramed=`0xff060000734e61507059`) is the only one that doesn't round-trip through `snappyframesz`.