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
