DevConsole: log streamer with file/tcp/stderr sinks

[meszmate]

Summary

Solves the perennial TUI debugging problem: stdout is owned by the renderer, so `std.debug.print` would garble the screen. `DevConsole` routes structured log events to a separate sink so a developer can run the TUI in one terminal and `tail -f` (or `nc`) the log stream in another. Modeled on `textual console`.

Sinks

Sink	Use it with
`.file`	`tail -f path.log` in another terminal
`.tcp`	`nc localhost ` — multiple viewers can connect simultaneously
`.stderr`	when stderr is redirected to a file or pipe

Multiple sinks can be added — events fan out to all of them.

API

```zig
var console = zz.DevConsole.init(allocator);
defer console.deinit();
try console.addSink(.{ .file = "/tmp/myapp.log" });
try console.addSink(.{ .tcp = .{ .port = 9999 } });
console.setMinLevel(.warn);

console.info("starting up", .{});
console.warn("retry {d}", .{n});
console.err("connection lost", .{});
```

Each event has a level (trace/debug/info/warn/err) and a timestamp. Mutex-guarded so it's safe to call from any thread. Distinct from the existing `Logger` which is a single-file convenience type without levels or fan-out.

Test plan

• `zig build test` — added 3 tests covering file sink output, min-level filtering, and level rank ordering
• Manual: `zig build run-someapp` in one terminal, `tail -f /tmp/myapp.log` in another, verify events appear
• Manual: `.tcp` sink with two simultaneous `nc` viewers