fix(verdict): report kernel fuel failures and document limits

[oxura]

Withdrawn: diagnostic-only proposal, not a kernel fix

This PR was closed unmerged after maintainer review. It does not resolve #1193 or #1322: shared graph reconstruction and unary Nat conversion remain kernel work. They stay open.

The proposed diagnostic format change is withdrawn; upstream keeps its current MISMATCH behavior when Bend accepts a term the kernel does not confirm. The overly broad WONTFIX entry has been removed from this branch in e7b0710. No kernel acceptance, fuel policy or evaluator/proof change was made.

The original before/after diagnostic experiments used a real kernel and remained fail-closed, but those observations are not evidence that either reported kernel limitation was fixed. This rejected proposal is not counted as resolving either issue.

[Lorenzobattistela]

Thanks @oxura, but we're going to close this one.

• It doesn't fix --verdict: the kernel runs out of fuel on conversion_shared_cells_once.bend, which bend2 checks by sharing (2.0.34) #1193 or --verdict: the kernel runs out of fuel on a closed Nat comparison of literals around 3 000 000 that bend2 checks at once (2.0.34) #1322. Both are kernel limits (shared graphs rebuilt as trees, unary Nat literals), and the kernel could in principle accept the same terms. So they stay open as kernel work.
• When the kernel runs out of fuel, Bend accepted something the kernel didn't confirm. That is a mismatch, so we want to keep the current MISMATCH output and not change the format for every kernel error.
• The WONTFIX entry is too broad. Whether those issues are won't-fix isn't something this PR should decide.

[oxura]

Understood. The diagnostic-only proposal does not fix #1193 or #1322; both remain open kernel work. I removed my broad WONTFIX entry in e7b0710 and corrected the PR description so it no longer claims to fix either issue. The format change is withdrawn with this closed, unmerged PR; upstream keeps MISMATCH when the kernel does not confirm an accepted term. I will not count this proposal as resolving either issue.