feat(wir): validate canonical interop contracts

[e54-bot]

Summary

• validate AssignMember targets as canonical memberAccess values, including native indexed member forms
• add canonical-WIR coverage for indexed variable operations and nested Break/Continue/Skip/Skip If
• document the Workshop-owned contract and return dictionary/switch lowering gaps to opy-rs

Issue

Fixes #123

Verification

• cargo fmt --all --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets
• cargo run -p workshop-rs --bin workshop-catalog-gen -- check
• git diff --check

All local gates passed. Hosted checks are not claimed until GitHub reports their final status.

Boundary

No OPY-specific WIR nodes or semantics were added. The dictionary literal and nested/multiple switch-shape lowering decisions remain owned by opy-rs; native Workshop indexed/member and control-flow primitives are now directly validated in workshop-rs.

[Teakowa]

crates/workshop-rs/src/validate.rs:is_member_assignment_target: the new valueInArray(memberAccess(...), index) branch accepts a target shape that AssignMember does not emit as a native member lvalue. AssignMember serializes the target through generic value emission, while parsed receiver.member[index] is already the 3-arg memberAccess shape covered by this PR. Restrict the accepted target to canonical memberAccess shapes, or add explicit valid native-lvalue emission plus round-trip evidence for the valueInArray form if it is intentionally part of the contract.