docs: add ecosystem cross-navigation links

docs/ECOSYSTEM.md

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+# Ecosystem Integration
+
+## Position in kcenon Ecosystem
+
+**thread_system** is the concurrency layer of the kcenon ecosystem. It provides high-performance thread pools and DAG-based task scheduling used across systems that require parallel execution.
+
+## Dependencies
+
+| System | Relationship |
+|--------|-------------|
+| common_system | Direct dependency — core interfaces and patterns |
+
+## Dependent Systems
+
+| System | How It Uses thread_system |
+|--------|--------------------------|
+| monitoring_system | Thread pool for metric collectors and background processing |
+
+## All Ecosystem Systems
+
+| System | Description | Docs |
+|--------|------------|------|
+| common_system | Foundation — interfaces, patterns, utilities | [Docs](https://kcenon.github.io/common_system/) |
+| thread_system | High-performance thread pool, DAG scheduling | [Docs](https://kcenon.github.io/thread_system/) |
+| logger_system | Async logging, decorators, OpenTelemetry | [Docs](https://kcenon.github.io/logger_system/) |
+| container_system | Type-safe containers, SIMD serialization | [Docs](https://kcenon.github.io/container_system/) |
+| monitoring_system | Metrics, tracing, alerts, plugins | [Docs](https://kcenon.github.io/monitoring_system/) |
+| database_system | Multi-backend DB (PostgreSQL, SQLite, MongoDB, Redis) | [Docs](https://kcenon.github.io/database_system/) |
+| network_system | TCP/UDP/WebSocket/HTTP2/QUIC/gRPC networking | [Docs](https://kcenon.github.io/network_system/) |
+| pacs_system | DICOM medical imaging (12 libraries) | [Docs](https://kcenon.github.io/pacs_system/) |
+
+## Ecosystem Overview
+
+See the [Ecosystem Overview](https://kcenon.github.io/common_system/) in common_system for the complete dependency map and system selection guide.