Add vars section to YAML schema and documentation

[networmix]

vars - YAML Anchors and Variables

The vars section provides a designated space for YAML anchor definitions. YAML anchors (&name) and aliases (*name) follow the YAML 1.1 specification and are processed by PyYAML during parsing, before NetGraph validation.

Anchor Types:

• Scalar anchors: Reference primitive values (strings, numbers, booleans)
• Sequence anchors: Reference arrays/lists
• Mapping anchors: Reference objects/dictionaries
• Merge keys (<<): Merge mapping properties with override capability

Minimal Example:

vars:
  default_cap: &cap 10000
  base_attrs: &attrs {cost: 100, region: "dc1"}

network:
  nodes:
    N1: {attrs: {<<: *attrs, capacity: *cap}}
    N2: {attrs: {<<: *attrs, capacity: *cap, region: "dc2"}}

Processing Behavior:

• Anchors are resolved during YAML parsing, before schema validation
• The vars section itself is ignored by NetGraph runtime logic
• Anchors can be defined in any section, not just vars
• Merge operations follow YAML 1.1 semantics (later keys override earlier ones)

[Copilot]

Pull Request Overview

This PR adds support for a vars section in YAML schema files to provide a designated space for YAML anchor definitions and variable reuse. The change enables users to define reusable values, attributes, and configurations using YAML anchors and aliases, following YAML 1.1 specification.

Key changes:

• Added vars section to JSON schema with optional object type
• Updated scenario parsing to recognize and allow the vars top-level key
• Added comprehensive test coverage for YAML anchor functionality

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
schemas/scenario.json	Adds vars property to JSON schema as optional object for anchor definitions
ngraph/scenario.py	Updates recognized keys list and docstring to include vars section
tests/test_schema_validation.py	Adds test to verify schema validation of vars section
tests/test_scenario.py	Adds comprehensive tests for YAML anchor and alias functionality
docs/reference/dsl.md	Documents vars section usage, anchor types, and processing behavior
docs/reference/api-full.md	Updates documentation generation timestamp