Fix upscale model loader export bootstrap

[pydn]

This PR fixes the UpscaleModelLoader export/runtime path for current ComfyUI builds and updates the docs to reflect the current install and export flow.

What changed

• initialize extra/custom nodes when a workflow references node types that are not yet loaded in the current runtime
• prefer COMFYUI_PATH and keep the resolved ComfyUI path at the front of sys.path
• defer loading NODE_CLASS_MAPPINGS until needed so exporter tests can inject fake mappings without importing the full ComfyUI runtime
• make generated scripts include the helper needed to resolve COMFYUI_PATH at runtime
• fall back to NODE_CLASS_MAPPINGS[...] for node classes that exist in the runtime but are not importable from a normal dotted module path
• treat mapping-based node initialization as requiring the custom-node import path

Regression coverage

• adds focused coverage for an upscale workflow using UpscaleModelLoader in tests/test_upscale_model_loader_export.py

Documentation updates

• rewrites the README around the current supported usage modes:
  • Web UI export via File -> Save As Script
  • CLI export from API-format workflows
• clarifies that Web UI support requires the repo to be discoverable through ComfyUI custom_nodes
• documents COMFYUI_PATH more clearly for CLI/exported-script usage
• updates screenshots/assets referenced by the README
• adds clearer troubleshooting guidance for:
  • missing Save As Script
  • ComfyUI Desktop prompt() limitations
  • ComfyUI path resolution
  • runtime model/path issues

Verification

• targeted regression passes locally:
  • uv run python -m unittest tests.test_upscale_model_loader_export -v

Scope

This PR intentionally includes the README/docs refresh already present on the branch along with the #143 fix. It does not include the separate runtime harness / E2E validation work or agent/spec scaffolding.