Lower OPY program structure into canonical Workshop WIR

[Teakowa]

Parent: #36
Depends on: #35

Goal

Complete the structural HIR→WIR mapping for the OPY program skeleton without expanding into the full statement/expression compatibility surface.

Scope

• Lower global/player variable declarations with reference-compatible index allocation and preserved source/name spans.
• Lower subroutine declarations and definitions into canonical workshop-rs subroutine/rule structures.
• Lower rules, disabled state, event identity/filter structure, conditions container, and source presentation metadata that belongs on canonical WIR nodes.
• Preserve OPY source-file/span provenance through every structural node and diagnostic.
• Use existing workshop-rs WIR/validation contracts directly; do not add another Workshop IR.
• Keep unsupported event/filter/annotation forms explicit as structured integration diagnostics.

Non-goals

• Full statement/expression lowering.
• Catalog-backed builtin/member/enum closure.
• Settings, locale, optimizer directives, post-compile hooks, or decompilation.

Acceptance criteria

• Declared variables/subroutines/rules map to structurally valid canonical WIR with deterministic allocation.
• Source/name spans remain usable after HIR→WIR conversion.
• Supported event structures validate; unsupported forms fail explicitly.
• opy-frontend remains Workshop-independent.
• Focused positive/negative integration fixtures and workspace CI are green.