feat: port selected p1 scenarios to java and python

[jsugg]

What

Ports two selected P1 teaching scenarios beyond the TypeScript stack: Challenging DOM to Java Selenium/TestNG and sortable tables to Python Playwright. Updates the scenario catalog, generated matrix, stack READMEs, and Java regression XML to match the new coverage.

Why

Proposal R19 calls for selective cross-stack ports only where the stack teaches a distinct tradeoff. Java demonstrates legacy UI rescue and Page Object modeling for difficult DOM surfaces, while Python demonstrates concise table data assertions.

Verification

• rtk python3 tools/check-scenarios.py
• rtk python3 tools/check-scenarios.py --write-matrix with stable docs/scenario-matrix.md hash
• rtk npx --yes markdownlint-cli2@0.23.0 README.md docs/scenario-matrix.md stacks/java-selenium-testng/README.md stacks/python-playwright/README.md
• cd stacks/python-playwright && rtk .venv/bin/ruff format --check . && rtk .venv/bin/ruff check . && rtk .venv/bin/mypy src tests
• THE_INTERNET_BASE_URL=http://localhost:7080 rtk .venv/bin/pytest tests/test_sortable_tables.py --browser chromium
• rtk docker run --rm -v "$PWD/stacks/java-selenium-testng":/work -w /work -v "$HOME/.m2":/root/.m2 maven:3.9.11-eclipse-temurin-25 mvn -q -DskipTests test-compile
• focused Java ChallengingDomTest passed through local Selenium Grid in Docker
• independent verifier-lite reran the focused Java Grid check successfully

Out of scope

No broad P1 parity expansion, dependency updates, or CI workflow changes.