crucible-mir: tolerant parsing of Unsupported instance shims

[AmeliaRose802]

Problem

mir-json emits {"kind": "Unsupported"} for six instance shim variants it does not yet model:

• ConstructCoroutineInClosureShim
• ThreadLocalShim
• FutureDropPollShim
• FnPtrAddrShim
• AsyncDropGlueCtorShim
• AsyncDropGlue

These appear in the intrinsics array of any compiled crate that exercises async Rust, thread-locals, or function-pointer-address taking — even when the user's verified function never touches those code paths.

Today the crucible-mir JSON parser at crucible-mir/src/Mir/JSON.hs:159 fails the entire mir_load_module call with "unsupported instance: Just (Object [\"kind\" .= \"Unsupported\"])" the moment it encounters one of these. This blocks SAW from verifying ordinary Rust code that happens to be in a crate or transitive dependency containing async machinery.

Fix

Add a new IkUnsupported Text constructor to InstanceKind and accept "kind": "Unsupported" in the JSON parser. The optional shim_kind payload carries the original mir-json shim name so any downstream consumer can produce a clear, actionable error if the shim is actually called during translation.

This is a tolerant-parse change: the module loads, the verified function (which doesn't reach the shim) succeeds, and a clear error is deferred to translation time should anyone ever try to compile or invoke the IkUnsupported instance.

Diff

crucible-mir/src/Mir/JSON.hs | 16 ++++++++++++++++
crucible-mir/src/Mir/Mir.hs  |  9 ++++++++-
2 files changed, 24 insertions(+), 1 deletion(-)

No behavior change for modules that don't contain Unsupported instances. Existing tests under crux-mir/test/conc_eval/ are not affected.

Companion PR

• GaloisInc/mir-json#272 — adds the shim_kind field to mir-json's Unsupported emissions so the deferred error message can name the specific shim. (This crucible-mir PR works fine without the mir-json companion — if shim_kind is absent, it falls back to "unknown".)

End-to-end validation

I ran this fix against the test in intTests/test_mir_async_basic/ (synchronous helper alongside async fn) plus a hand-crafted module containing an injected Unsupported entry, and confirmed:

1. ✅ Modules containing Unsupported instances load successfully (previously failed at parse time).
2. ✅ mir_verify on a function that doesn't reach the shim succeeds.
3. ✅ Genuinely unknown kind strings (e.g. "GarbageKind") still produce the original "unsupported instance: …" error — the fix is scoped to "Unsupported" only.

Filed as draft for upstream review.