Learn English – So if I don’t see you till then

I like it— I don't love it.

You lie — you're out.
The Punctuation Guide

I prefer the "em" dash for the described examples.
The examples probably should not be used in formal writing, except to report what somebody has said, or when attempting to get the reader's attention.

The above cite from The Punctuation Guide does not allow for the use of a "em dash" to replace a semi-colon. Generally it can replace parentheses:

I like it (I don't love it).

You lie (you're out).

That works for the first example, not the second.
I am stuck on the "em" dash.

The major difference between the predicates distinguished here with Wh-clause complements is that

• not know belongs to a class of predicates that takes a Disjunctive Wh-clause complement

whereas

• not care (in the US) belongs to a class that takes a Conjunctive Wh-clause complement.

The difference is in the pragmatic nature of the Wh-clause; conjunctive clauses are factive --
they presuppose the identity and truth of their complement. E.g,

• I am aware of/They'll be surprised by/He doesn't care what she does tomorrow.
  In this example, the clause refers to the set of all actions that she is to do tomorrow,
  as a settled matter; it's called conjunctive because the set consists of A and B and C and ...

while disjunctive clauses are indefinite, and presuppose nothing, E.g,

• It's a mystery/I wonder/They don't know what she'll do tomorrow.
  In this example, the clause refers to an unknown set of possible events or actions;
  it's called disjunctive because the set consists of A or B or C or ...

Since the conjunctive clause is presupposed, there's no need for a predictive modal like will. But the disjunctive clause is indefinite, and its truth set may be empty ("..., if anything"), so the predictive will is necessary.