Rust version pinning: cdm::import! calls execute latest — needs a CallBuilder calldata preamble upstream

Problem

Since #76, cdm.json pins an exact version per dependency and cdm::import! generates type-safe stubs from that version's exact ABI — but the generated calls go out as plain calldata, which the per-name proxy routes to the latest implementation. Rust consumers today get "pinned ABI, latest execution" (same gap as TypeScript #78 and Solidity #79, different blocker).

The on-chain side is deployed and e2e-proven: prefixing calldata with [MAGIC 0xa2264d53][u128 versionKey] executes exactly that version.

The blocker is upstream, and it's small

cdm::import! delegates all call-stub generation to pvm_contract_sdk::abi_import! (cargo-pvm-contract). Every generated .call(self) funnels into one function, CallBuilder::call_raw_inner in pvm-contract-core/src/call.rs, which is hardcoded to write [4-byte selector][ABI args] and immediately send:

input_buf[..4].copy_from_slice(&self.selector[..]);
self.payload.encode_to(&mut input_buf[4..]);
host.call_evm(flags, &address.0, gas, &value, input_buf, None)

There is no seam for a calldata prefix, and the workarounds are dead ends (verified during #76):

• a wrapper Host can't be injected — users pass self as the call context, whose host type is fixed by #[contract];
• an extension-trait .call silently loses method resolution to the inherent .call (worse than not existing: habit-typed calls would route to latest with no error);
• pre-sliced buffers don't help — call_raw_inner sends the same slice it writes.

Upstream ask (cargo-pvm-contract, ~30 lines, CDM-agnostic): an optional calldata preamble on CallBuilder — e.g. preamble: Option<&'static [u8]> (or fixed [u8; 20]) written before the selector in call_raw_inner/delegate_call_raw_inner, plus buffer-size accounting at abi_import's four 4 + encode_len() allocation sites. Inert when unset; useful to any proxy/envelope scheme, not just CDM.

CDM side, once the hook exists

• pvm_cdm::reference! already receives the package name at expansion time and cdm-macros already reads the pinned version from cdm.json — bake the packed key (major<<64|minor<<32|patch, constants mirrored in contract-registry-core::versioning) into the generated handle's preamble. User code doesn't change:

cdm::import!("@example/counter");
let counter = counter::Counter::cdm_lookup();
counter.increment().call(self)?;   // executes the pinned version, not latest

• A "latest" spec in cdm.json ⇒ no preamble (today's behavior, made opt-in).
• Same change enables the follow-on optimization: bake the stable proxy address at build time (cdm_from_env-style) and drop the per-call getAddress(string) registry round-trip from cdm_lookup() — the address is permanent per name now.
• Extend test:macro and proxy.e2e.test.ts with a pinned Rust consumer case.

Sequencing

1. Upstream PR/issue on paritytech/cargo-pvm-contract for the CallBuilder preamble (this issue's second section is paste-ready).
2. Once merged (deps float on branch = "main", so it flows in automatically): CDM-side wiring in pvm-cdm-macros + tests — no on-chain changes, the wire format has been live since #76.

References

• #76 (wire format + e2e proof of versioned routing), #78 (TypeScript counterpart), #79 (Solidity counterpart).
• reference-repos/cargo-pvm-contract: crates/pvm-contract-core/src/call.rs (call_raw_inner), crates/pvm-contract-macros/src/abi_import/mod.rs (buffer allocation sites).

Start with reference-repos/cargo-pvm-contract/crates/pvm-contract-core/src/call.rs and crates/pvm-contract-macros/src/abi_import/mod.rs, then review the CDM macro paths that generate handles. Verify the upstream preamble hook accounts for buffer sizing and remains inert when unset; then wire pinned versions in pvm-cdm-macros and extend test:macro and proxy.e2e.test.ts with a pinned Rust consumer case.