[blue fallback rolling] add minRemainingDebt config param

[claude]

Requested by peyha · Slack thread

Adds a minRemainingDebt config parameter: a partial roll must leave at least that much debt on Midnight. A full close-out is exempt, as for minRollableAssets.

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[peyha]

should we merge both require ?

[claude]

Both work — the difference is whether the two failures keep distinct errors.

Merged into one:

require(
    assets == debtAssets || (assets >= minRollableAssets && debtAssets - assets >= minRemainingDebt),
    RolledAssetsTooLow()
);

Or factored, keeping the errors apart:

if (assets != debtAssets) {
    require(assets >= minRollableAssets, RolledAssetsTooLow());
    require(debtAssets - assets >= minRemainingDebt, RemainingDebtTooLow());
}

I'd keep them apart: in the merged form RolledAssetsTooLow also fires when assets is too high, which reads wrong. Happy to push either — say which.

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[chrmatt]

Maybe more consistent to put || assets == debtAssets second in both cases if we don't do the if factoring?

[claude]

verify (Healthiness) is red here, and it is not this PR's.

certora/confs/Healthiness.conf verifies MidnightWrapper against Healthiness.spec; nothing under certora/ or rocq/ references BlueFallbackRolling, so the Certora scope cannot see anything this diff changes. The same leg is red on this PR's own base commit — main at fa842818, run 33181042673, 26m01s, the only failure in that run — and on #1168's head before it was merged (1h17m). Here it failed at 28m14s with no annotation beyond Process completed with exit code 1.

There is no fix to port: the job log is not retrievable outside the browser, and this matches the known nondeterministic Healthiness failure rather than a counterexample. I'll re-run this one job once the rest of the run finishes, and leave verify (NoDebtWithoutCollateral) alone.

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