[Core]: Memory type plugin reference stub

[rosspeili]

Component

other core

Kind

enhancement

Description

Reference role: memory type plugin per spec/type-plugin.contract.md — bind/teardown, in-memory stub backend, type.bound spine events. Proves adapter path for roadmap retention backends without Live ID or external vendors.

Code or session context (optional)

Parallel to brain stub (Brain type plugin stub). aura/core/registry.py registration; tests for bind with brain + memory stubs.