Emit slog attributes from every handler (closes #19, closes #24)

Handle never read the record's attributes, and WithAttrs and WithGroup
returned the receiver unchanged. The JSON and webhook handlers also marshaled
slog.Record itself, whose attributes are unexported.

Each handler now carries a handlerAttrs value, copied rather than mutated, so
sibling loggers cannot leak attributes into each other. JSON and webhook
output nest groups as objects; the console appends key=value pairs with dotted
group keys, quoted as slog.NewTextHandler quotes them, invalid UTF-8
included. Logged values are never written to. In JSON the record's own fields
win a key collision, and a repeated key keeps its last value unless both are
groups, which merge; the README documents both.

Deviation: DEL is quoted on the console; the stdlib leaves it bare.

Model: opus-5-5

console_handler.go

@@ -16,7 +16,9 @@ import (
 const callerSkipFrames = 4
 
 // ConsoleHandler writes human-readable, colored log lines to stdout.
-type ConsoleHandler struct{}
+type ConsoleHandler struct {
+	attrs handlerAttrs
+}
 
 // NewConsoleHandler returns a new ConsoleHandler.
 func NewConsoleHandler() *ConsoleHandler {
@@ -24,7 +26,8 @@ func NewConsoleHandler() *ConsoleHandler {
 }
 
 // Handle writes the record to stdout as a colored, timestamped line
-// including the caller file and line.
+// including the caller file and line, followed by the attributes as
+// key=value pairs.
 func (c *ConsoleHandler) Handle(
 	_ context.Context,
 	record slog.Record,
@@ -56,12 +59,13 @@ func (c *ConsoleHandler) Handle(
 	_, _ = fmt.Fprintln(
 		os.Stdout,
 		colorFunc(
-			"%s [%s] %s:%d: %s",
+			"%s [%s] %s:%d: %s%s",
 			timestamp,
 			record.Level,
 			file,
 			line,
 			record.Message,
+			attrsToText(c.attrs.forRecord(record)),
 		),
 	)
 
@@ -77,12 +81,22 @@ func (c *ConsoleHandler) Enabled(
 	return true
 }
 
-// WithAttrs returns the handler unchanged; attributes are not rendered.
-func (c *ConsoleHandler) WithAttrs(_ []slog.Attr) slog.Handler {
-	return c
+// WithAttrs returns a new handler that also emits attrs, qualified by the
+// groups open now. The receiver is not modified.
+func (c *ConsoleHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
+	if len(attrs) == 0 {
+		return c
+	}
+
+	return &ConsoleHandler{attrs: c.attrs.withAttrs(attrs)}
 }
 
-// WithGroup returns the handler unchanged; groups are not rendered.
-func (c *ConsoleHandler) WithGroup(_ string) slog.Handler {
-	return c
+// WithGroup returns a new handler that qualifies later attributes with
+// the group name. An empty name returns the receiver.
+func (c *ConsoleHandler) WithGroup(name string) slog.Handler {
+	if name == "" {
+		return c
+	}
+
+	return &ConsoleHandler{attrs: c.attrs.withGroup(name)}
 }