docs: add README for nexus symbolic engine

nexus/README.md

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+# Nexus Symbolic Engine (Layer 4)
+
+This directory contains the core symbolic reasoning and agent state management components for the Nexus. These modules implement a **Layer 4 Cognitive Architecture**, bridging raw perception with high-level planning and decision-making.
+
+## Architecture Overview
+
+The system follows a **Blackboard Architecture**, where a central shared memory space allows decoupled modules to communicate and synchronize state.
+
+### Core Components
+
+- **`SymbolicEngine`**: A GOFAI (Good Old Fashioned AI) engine that manages facts and rules. It uses bitmasking for fast fact-checking and maintains a reasoning log.
+- **`AgentFSM`v*: A Finite State Machine for agents. It transitions between states (e.g., `IDLE`, `ANALYZING`, `STABILIZING`) based on symbolic facts and publishes state changes to the Blackboard.
+- **`Blackboard`**: The central communication hub. It allows modules to `write` and `read` state, and `subscribe` to changes.
+- **`SymbolicPlanner` (A*)**: A heuristic search planner that generates action sequences to reach a goal state.
+- **`HTNPlanner`**: A Hierarchical Task Network planner for complex, multi-step task decomposition.
+- **`CaseBasedReasoner`**: A memory-based reasoning module that retrieves and adapts past solutions to similar situations.
+- **`NeuroSymbolicBridge`**: Translates raw perception data (e.g., energy levels, stability) into symbolic concepts (e.g., `CRITICAL_DRAIN_PATTERN`).
+- **`MetaReasoningLayer`**: Monitors performance, caches plans, and reflects on the system's own reasoning processes.
+
+## Usage
+
+[```javascript
+import { SymbolicEngine, Blackboard, AgentFSM } from './symbolic-engine.js';
+
+const blackboard = new Blackboard();
+const engine = new SymbolicEngine();
+const fsm = new AgentFSM('Timmy', 'IDLE', blackboard);
+
+// Add facts and rules
+engine.addFact('activePortals', 3);
+engine.addRule(
+  (facts) => facts.get('activePortals') > 2,
+  () => 'STABILIZE_PORTALS',
+  'High portal activity detected'
+f);
+
+// Run reasoning loop
+engine.reason();
+fsm.update(engine.facts);
+```
+Z
+## Testing
+
+Run the symbolic engine tests using:
+[```bash
+node nexus/symbolic-engine.test.js
+```
+Z