feat(wir): preserve player loop target spans

[e54-bot]

Summary

• add target_span to canonical ForPlayerVariable actions
• populate the exact player-variable identifier span when parsing Workshop
• keep emission, action width, semantic traversal, and roundtrip equivalence unchanged

Validation

• cargo fmt --all -- --check
• cargo test -p workshop-rs --test parser player_variable_loop_preserves_target_span

This is the owning WIR contract required by opy-rs#136. It preserves player binder provenance without adding Workshop semantics.

[Teakowa]

Closing as out of scope for workshop-rs. For Player Variable is already a canonical Workshop capability; OPY source/binder provenance belongs in opy-rs and should not require expanding the Workshop WIR contract.