Update to v0.16.0 (#104)

* Update to v0.16.0

- Reorganized modules: `ngraph.exec` split into `ngraph.analysis` and `ngraph.model`, maintaining public API via re-exports.
- Expanded public API to include `TrafficDemand`, `FlowPolicyPreset`, `Scenario`, `NetworkExplorer`, and placement functions.
- Introduced `ngraph.analysis.placement` for demand placement with SPF caching, including `place_demands()` and `PlacementResult`.

* Refactor execution counter handling for thread safety

Author: networmix

CHANGELOG.md

@@ -5,6 +5,20 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.16.0] - 2025-12-21
+
+### Changed
+
+- **Module reorganization**: `ngraph.exec` split into `ngraph.analysis` (runtime analysis) and `ngraph.model` (data structures); public API unchanged via re-exports
+- **Expanded public API**: `TrafficDemand`, `FlowPolicyPreset`, `Scenario`, `NetworkExplorer`, and placement functions now exported from top-level modules
+- **Placement analysis**: Extracted SPF caching and demand placement logic into `ngraph.analysis.placement` module with `place_demands()` and `PlacementResult`
+
+### Added
+
+- `ngraph.model.demand` subpackage: `TrafficDemand` and builder functions
+- `ngraph.model.flow` subpackage: `FlowPolicyPreset` and policy configuration
+- `ngraph.types` exports: `Mode`, `FlowPlacement`, `EdgeSelect`, `EdgeRef`, `MaxFlowResult`
+
 ## [0.15.0] - 2025-12-21
 
 ### Added