`pformat_boxed_tb()` has never accepted an `indent` kwarg but
`pformat_caller_frame(box_tb=True)` has been passing one since
`888af602`. Nothing in the suite covered the branch, so the
`TypeError` only ever surfaced from `_mk_send_mte()` — i.e.
EVERY send-side `MsgTypeError` blew up while formatting itself
and masked the real msg-spec violation behind a bogus
`TypeError`.
Red on purpose per the test-first convention; the 1-line fix
lands next.
Also pin `pformat_boxed_tb()`s signature so a future typo'd
kwarg fails loudly at the call site instead of only when some
rare error path runs.
(this patch was generated in some part by `claude-code` using `claude-opus-5` (`anthropic`))
Gud Boi