[codex] expose low-level api and harden release gate

[artpods56]

Summary

• expose the public low-level ksef2.raw API and raw auth mappers
• add ksef2.xades as the public facade for certificate and XAdES helpers
• replace stale roadmap shards with the current 1.0 readiness checklist
• document raw API, public API boundaries, error handling, and sync generation
• remove legacy sample XML helpers/assets and use FA(3) builder-based samples
• make CI and PyPI publishing use just release-check
• move the branch to 0.19.0 because v0.18.1 is already tagged on main

Validation

• just release-check produced dist/ksef2-0.19.0.tar.gz and dist/ksef2-0.19.0-py3-none-any.whl
• just integration
• just e2e
• uv run python scripts/validate_docs_frontmatter.py docs/
• uv run --extra pdf python -m pytest tests/integration/test_examples.py::test_example_batch_export_to_pdf -m integration -q
• docs repo: npm run sync-docs -- --only sdk && npm run build

Remote checks

• GitHub ci (3.12) passed for 7c3a320
• GitHub ci (3.13) passed for 7c3a320
• GitHub coverage passed for 7c3a320
• GitHub docs validate passed for 7c3a320

Release notes

This PR moves the SDK into 1.0-readiness territory, but it does not cut the 1.0.0 release. Final tagging should still happen from a clean tree after this PR and the paired docs PR are merged, with TEST integration repeated immediately before the tag.

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[artpods56]

Temporarily closing and reopening to refresh pull_request checks for the latest head commit.