Lower OPY statements and expressions into canonical Workshop WIR

[Teakowa]

Parent: #36
Depends on: #40 (completed)

Goal

Close the declared non-catalog OPY statement/expression lowering surface into canonical workshop-rs WIR with explicit, evidence-backed handling of unsupported semantics.

This issue is now an acceptance gate rather than a single implementation PR.

Execution issues

• Lower OPY assignment and expression primitives into canonical Workshop WIR #46 — Lower OPY assignment and expression primitives into canonical Workshop WIR
• Lower OPY control flow into canonical Workshop WIR with oracle evidence #47 — Lower OPY control flow into canonical Workshop WIR with oracle evidence

Intended order: #46 → #47. Keep both PRs independently reviewable.

Scope

• Complete assignment/modification, variable/value, indexing/formatting/operator, and source-level no-op handling through Lower OPY assignment and expression primitives into canonical Workshop WIR #46.
• Complete if, while, range for, doWhile, switch, and break semantics through Lower OPY control flow into canonical Workshop WIR with oracle evidence #47.
• Preserve provenance on produced WIR and structured integration diagnostics.
• Use existing canonical WIR primitives; normalize higher-level OPY constructs only when pinned observable evidence justifies semantic equivalence.
• Keep receiver/member/enum/catalog closure assigned to Close catalog-backed OPY lowering and corpus evidence #42.

Non-goals

• Closing the complete builtin/member/enum catalog surface (Close catalog-backed OPY lowering and corpus evidence #42).
• Settings, locale, optimizer directives, post-compile hooks, or reconstruction.
• Output-text identity with OverPy.
• Adding OPY-shaped nodes to workshop-rs merely to mirror the source HIR.

Acceptance criteria

• Lower OPY assignment and expression primitives into canonical Workshop WIR #46 and Lower OPY control flow into canonical Workshop WIR with oracle evidence #47 are complete with independent oracle-backed implementation/test evidence.
• Every in-scope statement/expression HIR variant either lowers correctly or fails with a stable, source-attributed integration diagnostic.
• Produced WIR passes structural validation.
• Switch/fallthrough/break and loop semantics are backed by pinned evidence rather than implementation convenience.
• No silent node dropping, guessed WIR, or temporary Workshop IR is introduced.
• Completion leaves Close catalog-backed OPY lowering and corpus evidence #42 as the only remaining Lower the declared OPY semantic surface into canonical Workshop WIR #36 implementation stage.