Add extensible environment variable mapping system

[roxblnfk]

What was changed

Added extensible environment variable mapping system for view templates. Applications can now define custom environment constants through registry entries instead of modifying framework code.

Example:

- name: app_constants
  kind: registry.entry
  meta:
    type: view.env_mapping
    priority: 25
  mappings:
    hostname: APP_BASE_URL
    api_url: PUBLIC_API_URL
    debug_mode: DEBUG_MODE

These become available in templates as {{ env.hostname }}, {{ env.api_url }}, etc.

Why?

Eliminates hardcoded environment mappings in the framework. Applications can add their own constants without touching framework code. Priority system (0-100) allows proper configuration layering between framework defaults, system overrides, and application-specific settings.