feat: add progression readiness api

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

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+# Phase 4 Plan: Progression
+
+**Status:** Ready for execution
+**Phase Goal:** Show evidence-backed readiness and select the next challenge.
+
+## Requirements Covered
+
+- PROG-01: User can see a role-specific readiness map.
+- PROG-02: Concepts have challenge ladders from definition to interview
+  pressure.
+- PROG-03: System selects next challenge based on learner memory and grading
+  evidence.
+- PROG-04: System unlocks boss-style integrated questions after prerequisite
+  stability.
+- PROG-05: Streaks and rewards avoid punitive or gambling-like mechanics.
+
+## Tasks
+
+### 1. Add progression package
+
+- Define readiness map, concept progress, reward, and unlock types.
+- Compute readiness from learner memory snapshots.
+
+### 2. Add next challenge selection
+
+- Select the weakest evidenced concept first.
+- Use review schedule or misconception evidence to choose recovery difficulty.
+- Produce typed `workflows.NextChallenge`.
+
+### 3. Add HTTP endpoints
+
+- `GET /api/v1/learners/{userID}/readiness-map`
+- `GET /api/v1/learners/{userID}/next-challenge`
+
+### 4. Add tests and verification
+
+- Test readiness map projection.
+- Test next challenge selection from weak memory.
+- Test HTTP readiness flow after diagnostic answer.
+- Run Go tests, OpenSpec validation, line-count check, and smoke.
+
+## Out of Scope
+
+- Frontend visualization.
+- Persistent streak history.
+- Multi-track progression graph.