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. 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: a key containing invalid UTF-8 stays unquoted, unlike the stdlib:
#24.
Model: opus-5-5 (rebase, this message)
