3-component re-exporter resource chains: implement shim module approach

[avrabe]

Context

Epic #69 has been extensively investigated across multiple sessions. 70/73 runtime tests pass. The remaining 3 fixtures (resource_floats, resource_with_lists, resource-import-and-export) are 3-component re-exporter chains that fuse successfully but crash at runtime.

Root cause (proven)

The re-exporter's generated code (wit-bindgen) has conflicting requirements for borrow params:

1. Dereference: The code uses the value directly as a raw pointer (*(value as *const FloatRep)) — needs the REP
2. Drop: Rust's Drop impl calls resource.drop(value) — needs a valid wasmtime HANDLE

With shared wasmtime resource types, these are incompatible: REPs are memory addresses (large), handles are sequential integers (small). No adapter-level fix can make one value serve both purposes.

Solution: Re-exporter shim module

Instead of trying to replicate canon lift/lower in the adapter, generate a proper shim module for re-exporter boundaries that preserves the component model's resource table semantics.

The shim module would:

1. Export the re-exporter's functions with proper canon lift wrapping
2. Handle borrow handle creation/destruction in the re-exporter's resource table
3. Convert between the caller's and callee's resource representations
4. Be instantiated as a separate core module within the P2 wrapper

This is the approach used by wit-component (the reference implementation) for re-exported resources.

Files to modify

• merger.rs: Export re-exporter's internal functions from the fused module
• resolver.rs: Skip adapter creation for re-exporter-targeted calls, mark them for shim handling
• component_wrap.rs: Generate the shim module with canon lift/lower for re-exporter functions
• lib.rs: Skip wire_adapter_indices for shim-handled calls

Key findings from investigation

• resource.rep in the re-exporter is identity (the generated code doesn't call wasmtime's resource.rep)
• The re-exporter stores inner handles from canon lower in FloatRep.inner — these must be memory addresses (from the re-exporter's resource table), not sequential integers
• [method] functions extract inner handles via double indirection (struct field access); [static] functions forward reps directly from the stack — different indirection levels through the same 3→0 adapter
• The cleanup path (function 417 → 353 → resource.drop) drops values via the imported type's resource.drop, which must receive valid handles

Related PRs

• fix: reject multiply-instantiated modules in resolver #76-fix: prevent resource map overwrites and remove unsafe graph upgrades #91: Infrastructure improvements (all merged)
• Specifically fix: prevent resource map overwrites and remove unsafe graph upgrades #91: Safe fixes (or_insert + downgrade-only graph override)

Labels

Epic #69, enhancement