feat: add progression readiness api

.planning/phases/004-progression/004-RESEARCH.md

@@ -0,0 +1,32 @@
+# Phase 4 Research: Progression
+
+## Findings
+
+Learner memory already stores the minimum evidence needed for progression:
+
+- concept mastery state
+- evidence references
+- misconceptions
+- review schedules
+- interventions
+
+The MVP progression surface can therefore be computed as a projection rather
+than a new durable source of truth.
+
+## Recommended Shape
+
+- `internal/progression` owns readiness projection and challenge selection.
+- `learnermemory.Service` remains the source for learner state.
+- Readiness percentage should be simple and explainable.
+- Challenge ladder should map readiness state to the next useful task:
+  - unknown/fragile: define or recovery
+  - improving: tradeoffs
+  - interview-ready: design constraints
+  - strong signal: interview pressure
+- Boss unlock requires at least two stable concepts with evidence.
+
+## Risks
+
+- Too much gamification logic can become speculative. Keep it deterministic.
+- Readiness percentages can feel fake if not traceable. Include evidence.
+- Missing memory should return a normal 404, not invented progress.