[codex] prepare 0.6 release

[ObservedObserver]

Summary

Prepare the PyGWalker 0.6 release line without removing deprecated 0.6-compatible APIs.

• Bump pygwalker.__version__ to 0.6.0rc0.
• Add docs/RELEASE_0_6.md with the public compatibility policy, computation API direction, anywidget-first Jupyter behavior, static HTML limits, privacy defaults, and tested runtime support.
• Link the 0.6 release notes from the README.
• Tighten release-readiness tests for legacy computation flags, public cloud_computation=True compatibility, version metadata, release notes, and publish workflow readiness.
• Fix packaging so wheel-from-sdist can rebuild the frontend and so wheel artifacts do not include frontend build-time app/ or node_modules content.

Root Cause / Packaging Fix

The release build surfaced two package hygiene issues:

• The sdist missed app/src/lib, because the repo-level Python .gitignore has a generic lib/ pattern. The sdist now force-includes app/src/lib.
• The wheel-from-sdist build installed frontend dependencies, and the previous Hatch include pattern for bin could match nested node_modules/*/bin files. Build includes are now root-anchored.

Validation

• venv/bin/python -X faulthandler -W error::DeprecationWarning:pygwalker -m pytest -o faulthandler_timeout=300 tests -> 317 passed
• ruff check pygwalker tests scripts bin pygwalker_tools
• ruff format --check pygwalker tests scripts bin pygwalker_tools
• generated app/src/interfaces/comm.generated.ts freshness check
• ./scripts/compile.sh
• yarn playwright install chromium && yarn test:front_end -> 1 passed
• venv/bin/python -m build .
• venv/bin/python -m twine check dist/*
• Wheel inspection: includes pygwalker/templates/dist/pygwalker-app.iife.js; excludes app/, node_modules, tests, caches, .DS_Store, and old wheels.

Independent Review

• Compatibility/package audit sub-agent reviewed the 0.6 gaps and identified the legacy computation flag symmetry and public cloud compatibility tests, both addressed here.
• Independent acceptance agent used the Codex Chrome Extension with real Chrome to open a locally generated PyGWalker page. The Graphic Walker UI rendered with Data / Visualization, fields, toolbar, and shelf drop zones visible, with no console errors or warnings.