SPIKE: Protocol 28 (CAP-0084)

[sisuresh]

Changes

• Add SC_ADDRESS_TYPE_MUXED_CONTRACT enum arm + MuxedContract { uint64 id; ContractID contractId; } struct + SCAddress union arm to Stellar-contract.x, all gated behind #ifdef CAP_0084_MUXED_CONTRACT, mirroring the existing MuxedEd25519Account muxed-account pattern.
• Gated, so the new types appear only in the next branch (built --all-features) and stay out of curr until core bumps max protocol to 28. curr/next regenerate automatically on merge — not edited here.
• Stellar-ledger.x comment revert (deliberate — reviewer please note): drops the unrelated Fix conflicting comment on rent fee #304 comment change so the p28 SPIKE XDR base stays byte-identical to the frozen p27 host (ledger.x hash 93cdd4dd), avoiding downstream host-identity drift.

Flag-token contract (downstream)

This repo is the root of the chain — no upstream. Downstreams pin this PR's head commit by full 40-char SHA: 787382ef2099cca168ca1cb282730d6b7b9e2f16 (never the truncated 787382e). The ifdef token CAP_0084_MUXED_CONTRACT is the verbatim contract for every downstream:

• cargo feature (rs-stellar-xdr / rs-soroban-env / js-xdr-json): cap_0084_muxed_contract
• XDR_FEATURES / xfile preprocess --features (go-stellar-sdk / js-stellar-base): CAP_0084_MUXED_CONTRACT

A mismatched/truncated token silently strips the new types downstream.

Verification

Local stellar-xdr xfile preprocess: arm absent with no features (curr); all three arms present and file parses with --features CAP_0084_MUXED_CONTRACT.

Deferred

• Downstream PRs (rs-soroban-env/-sdk, stellar-core, go-stellar-sdk, horizon, rpc, js-stellar-base/-sdk, js-stellar-xdr-json, laboratory, docker) pin this PR's head SHA — opened by their own per-repo passes.

Downstream: stellar/rs-stellar-xdr#551 (pins this PR head 787382e)
CAP-0084: stellar/stellar-protocol#1968