Add Phase 84 context memory suite stability plan

[pcharbon70]

Summary

• Add Phase 84 planning for context-memory test isolation and mixed-suite stability.
• Capture the known ETS cleanup race as a bounded test-lifecycle problem.
• Update the planning index and chronology note so Phase 84 follows Phase 83.

Verification

• git diff --check

Notes

• bw prime and bw ready could not run because bw is not installed in this checkout.