Custom sinks cannot render message templates - internal/parser is not accessible

[willibrandon]

Version: Latest (main branch)

What happened?
Custom sinks cannot properly render message templates because internal/parser is not accessible outside the mtlog module. When implementing core.LogEventSink, there's no public API to render the MessageTemplate with Properties, forcing manual string replacement that breaks with capturing operators (@), scalar hints ($), and format specifiers.

Expected behavior
Custom sinks should be able to render message templates just like built-in sinks. Serilog provides LogEvent.RenderMessage() for this purpose. mtlog should provide equivalent functionality.

Minimal reproduction

package main

import (
    "fmt"
    "os"
    "strings"
    "github.com/willibrandon/mtlog"
    "github.com/willibrandon/mtlog/core"
)

// Custom sink per README documentation
type CustomSink struct{}

func (s *CustomSink) Emit(event *core.LogEvent) {
    // ❌ No way to properly render the message!
    // event.MessageTemplate = "Config: {@Config}"
    // event.Properties = {"Config": &config.Config{...}}
    
    // Can't do this (internal package):
    // tmpl, _ := parser.Parse(event.MessageTemplate)
    // message := tmpl.Render(event.Properties)
    
    // Only option: manual string replacement (broken):
    message := event.MessageTemplate
    for name, value := range event.Properties {
        // Doesn't handle {@Property}, {$Property}, {Property:format}, etc.
        placeholder := fmt.Sprintf("{%s}", name)
        message = strings.ReplaceAll(message, placeholder, fmt.Sprintf("%v", value))
    }
    
    fmt.Fprintln(os.Stderr, message)
}

func main() {
    logger := mtlog.New(mtlog.WithSink(&CustomSink{}))
    cfg := map[string]any{"debug": true, "port": 8080}
    
    // Outputs: "Config: {@Config}" (placeholder not replaced!)
    logger.Debug("Config: {@Config}", cfg)
}

Environment

• Go version: go1.25.1
• OS: darwin/arm64

Additional context
Serilog solves this by providing LogEvent.RenderMessage() and public MessageTemplateTextFormatter. Suggested solution:

Add RenderMessage() method to core.LogEvent (Serilog-style, cleanest)

Current workaround in use: Manual replacement of {Property}, {@Property}, and {$Property} with strings.ReplaceAll, but this doesn't support format specifiers or alignment.