Normalize tests around feature ownership

[Teakowa]

Goal

Make the opy-rs test suite reflect stable language and tooling features rather than GitHub work-item boundaries.

Context

The current suite contains durable test files and modules organized around Issue identifiers. Issues are planning containers: a single Issue may represent one defect, a broad capability block, or a collection of related gaps. That makes Issue identity unsuitable as a long-lived test taxonomy and obscures which feature each test actually belongs to.

Issue, PR, and task identifiers may remain as provenance, but every durable test must belong to a stable feature. Contracts, invariants, regressions, and failure modes are tested within that feature ownership.

Scope

• Audit durable opy-rs tests whose naming or placement is organized around Issue/PR/task identifiers.
• Assign every retained case to its owning language, compiler, or tooling feature.
• Reorganize test files, modules, suites, case names, and fixtures around those stable feature owners and the behavior protected within them.
• Preserve useful provenance for real-project and historical regressions without using provenance as taxonomy.
• Consolidate equivalent coverage where the current Issue-oriented layout duplicated the same feature contract.

Non-goals

• Changing OPY semantics, compiler behavior, diagnostics, or support claims solely as part of the reorganization.
• Adding tests merely because files are being moved or renamed.
• Removing provenance-linked regression evidence that still protects a distinct feature behavior or failure mode.
• Redesigning unrelated test infrastructure.

Acceptance criteria

• No durable opy-rs test file, module, suite, case, or committed test directory is owned or named primarily by an Issue, PR, or task identifier.
• Every retained test has a clear stable feature owner.
• Contracts, invariants, regressions, and failure modes are represented under their owning feature rather than as independent work-item test categories.
• Broad historical Issues no longer aggregate unrelated feature tests into one Issue-owned suite.
• Useful Issue/PR and real-project provenance remains available in comments or fixture metadata where relevant.
• Equivalent coverage is not duplicated solely to preserve historical Issue boundaries.
• Existing semantic, compatibility, corpus, and failure expectations are not weakened by the reorganization.

Dependencies / ownership

• Test taxonomy and evidence policy: Require feature-owned test taxonomy .github#36.
• OPY language and compatibility test ownership remains in opy-rs.