[EM] Manipulability stats for (some) poll methods

[Closed Limelike Curves]

Hi Kris, thanks for the results! They're definitely interesting.

That said, I'm not sure how useful a metric raw probabilities provide; I
don't think they provide a very strong measure of how *severely* each
system is affected by strategy. Missing are:
1. How much do voters have to distort their ballots? Is it just truncation,
compression (as with tied-at-the-top), or full decapitation?
2. How hard is it to think of the strategy? Counterintuitive strategies
(e.g. randomized strategies or pushover) require large, organized parties
to educate their supporters about how to pull it off. This could be good or
bad depending on if you like institutionalized parties. Good: sometimes
people can't pull it off. Bad: this creates an incentive for strong parties
and partisanship. See the Alaska 2022 Senate race, where Democrats pulled
off a favorite-betrayal in support of Murkowski to avoid a center-squeeze.
3. Is a counterstrategy available?
4. How feasible is the strategy (does it involve many or few voters)?
5. How bad would the effects of the strategy be? Borda is bad not just
because it's often susceptible to strategy, but because it gives turkeys a
solid chance of winning.

IRV is a good example of this. It's *usually* not susceptible to strategy
(in the IAC model), but I think of it as one of the most strategy-afflicted
methods on this list. It's vulnerable to some particularly-egregious
strategies (decapitation), ones that are complex or difficult to explain
(pushover), and many strategies don't have a simple defensive
counterstrategy available (like truncation).

A low-probability but occasionally high-impact strategy might be the worst
of both worlds; voters get lulled into a false sense of security by a few
elections where strategy doesn't matter, then suddenly find a candidate
they dislike elected because they failed to execute the appropriate
defensive strategy.
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