Title

Define phased retirement plan for compatibility-only runtime/compiler paths

Problem

Compatibility paths are intentionally retained, but retirement criteria/timeline are not yet formalized for maintainers and contributors.

Why It Matters

• Reduces long-term maintenance overhead.
• Improves clarity on canonical execution path ownership.
• Prevents accidental feature work on compatibility-only surfaces.

Acceptance Criteria

• Inventory compatibility-only paths (runtime wrappers, legacy IR paths, related docs).
• Define phase gates for retirement (usage signals, test coverage, docs updates).
• Add explicit "do not extend semantics here" guidance where applicable.
• Publish a short deprecation/retirement timeline in docs.

Non-Goals

• Immediate removal of compatibility paths.
• Breaking historical imports without migration notes.
• Architecture rewrites unrelated to retirement clarity.