Debugging: add unique-within-store IDs for every entity.

[cfallin]

When building an introspection API for use by a debugger, we need a way to expose identity: that is, to give some way of knowing that a given Memory, Instance, etc. is this one and not that one. Our handle types variously have either Eq implementations or e.g. Module::same for the ones that wrap an Arc under-the-covers; but that's not enough to allow a debugger to e.g. build a hashmap for whatever metadata that it might expose via whatever debugging protocol.

For maximal generality and flexibility, we should expose the unique IDs that already more-or-less exist: for instances, that is their instance ID directly; for entities owned by instances, we can build a u64 with the instance ID in the upper 32 bits and the defined-index in the lower 32 bits. IDs for all entities except modules are unique-within-a-Store (and this is all that is needed); IDs for modules happen to reuse the CompiledModuleId and so are unique-within-an-Engine.

I've opted to name these debug_index_within_store to scope the feature and intended use-case clearly, but if there's a desire, I could easily rename them to simply index. I shied away from that here because I didn't want to give a notion that these indices are somehow canonical or correspond to some order or other.

[cfallin]

(I'll note as an aside that this is I believe the last PR I need to the wasmtime crate to build a functional debugger top-half; all the rest of my work is in the debugger crate and the guest debugger component. Thanks for all the reviews so far!)

[cfallin]

Just to record CI breakage for the record: this job in the merge queue is failing with

2026-02-24T17:57:36.054084Z  WARN oci_client::token_cache: Invalid bearer token error=Error(InvalidToken)
Error: registry error: HTTP status server error (503 Egress is over the account limit.) 

when trying to fetch WITs from a registry. I'll retry and see if it's ephemeral or if the limit reached is real.

[cfallin]

It's failing again here -- cc @alexcrichton @pchickey, it looks like we have CI blocked (just after all the security releases merged fortunately?) from wkg hitting an account limit. Should we delete the check for now?

[alexcrichton]

Keep throwing it at the queue, we'll deal with the root cause later.