Align instability documentation with current runtime (#74)

README.md

@@ -8,9 +8,13 @@ The bootstrap, books/linking, Ages, writing-system, and Age-grammar milestones a
 The writing system uses a data-pack symbol catalog and persistent
 ordered-writing model. Linking Books and Descriptive Books create persistent
 Ages from either authored supported symbols or seeded random completion.
-Instability and the remaining unsupported Legacy symbol effects have not been
-ported yet. The [Milestone 5 acceptance record](docs/MILESTONE_5_ACCEPTANCE.md)
-lists the supported grammar, optional integrations, and limits.
+Milestone 6 instability work is in progress: Ages persist frozen instability
+scores and provider plans, and the bounded runtime implements status effects,
+colored decay, lightning, explosions, meteors, burning, and erosion. The
+[Milestone 5 acceptance record](docs/MILESTONE_5_ACCEPTANCE.md) lists the
+supported grammar, optional integrations, and limits; the
+[instability runtime policy](docs/INSTABILITY_RUNTIME.md) records the current
+effects and modern safety boundaries.
 
 Milestone 4 now includes stackable blank Pages plus server-created symbol,
 unresolved-symbol, and baseline link-panel states. Page visuals follow the