fix(server): replace readSliceShort with posix.read to fix HTTP hang

[YeonV]

Problem

After the Zig 0.16 migration (#5 / 54fe635), readHttpRequest reads incoming data using reader.interface.readSliceShort():

var reader = stream.reader(std_compat.io(), &read_buffer);
const n = try reader.interface.readSliceShort(&chunk);

readSliceShort loops until the entire chunk buffer (4096 bytes) is completely filled before returning. Since a typical HTTP request is far smaller than 4096 bytes, the socket read blocks indefinitely — the client sends its request, nullboiler never responds, and the connection times out.

Fix

Replace readSliceShort with std.posix.read, which is a single-syscall partial read that returns immediately with however many bytes are available:

const n = std.posix.read(stream.socket.handle, &chunk) catch return error.ReadFailed;

This is the correct semantics for an incremental HTTP request reader, and is consistent with how the pre-0.16 code used stream.read(&chunk) directly.

Impact

Without this fix, every HTTP request to nullboiler hangs — the server accepts the TCP connection but never sends a response. The process appears healthy but is completely non-functional.