Complete OPY-to-canonical-WIR lowering for the supported source language

[Teakowa]

Parent: #88

Goal

Close the remaining compiler/lowering gaps for the supported OverPy source-language surface as one category-level capability, so accepted OPY constructs consistently lower through canonical workshop-rs WIR without one-gap-per-issue execution.

Context

Several currently open issues describe narrow lowering differences such as initializer behavior, indexed assignment, string representation/modifiers, contextual chase lowering, and implicit subroutine materialization. These are useful regression evidence, but they should be completed through one audited lowering contract rather than as independent roadmap slices.

Scope

• Audit every supported OPY HIR construct that is expected to compile to canonical WIR and verify that its lowering path is implemented or has an explicitly owned external blocker.
• Close demonstrated lowering gaps for declarations/defaults, assignments/indexing, strings and collections, control flow, subroutines/functions, contextual helpers, builtin/member calls, rules/events, and other supported source constructs.
• Preserve source provenance and structured diagnostics across lowering.
• Compare observable/canonical semantics rather than compiler-output text, optimizer shape, helper naming, or incidental WIR differences that are not part of the contract.
• Run category-level conformance plus the full real-project corpus after the lowering surface is complete.

Non-goals

• Reproducing upstream optimizer strategy, temporary variables, formatting, or byte/text-identical Workshop output.
• Duplicating canonical Workshop semantics owned by workshop-rs.
• Workshop -> OPY reconstruction.
• Project-specific lowering workarounds.

Acceptance criteria

• Every supported source/HIR construct expected to compile has an audited OPY -> canonical WIR path or an explicit owner blocker.
• Existing demonstrated lowering regressions from Track OverPy null-initializer default semantics #58, Define bounded multidimensional indexed-assignment lowering #60, Lower literal OverPy string modifiers #104, Preserve canonical Custom String representation for literal values #105, Complete contextual chase keyword lowering #106, and the lowering portion of Materialize implicit subroutine declarations for def definitions #107 are covered by the category-level contract.
• Supported cases preserve observable semantics and source provenance through canonical validation/emission.
• Full-corpus execution no longer exposes an unowned OPY lowering capability inside the declared source-language compatibility target.
• Accepted non-semantic output-shape differences are not converted into compatibility blockers.

PR budget

Target: 1 implementation PR. Maximum: 2 non-release PRs. Split only by coherent lowering boundaries, not by individual residual or fixture.