feat(py): implement p0 scenarios (1/2)

[jsugg]

What

Adds the first Python P0 scenario batch: add/remove elements, checkboxes, dropdown, inputs, JavaScript alerts, dynamic loading, and dynamic controls. Updates catalog coverage, generated matrices, and Python stack docs for the new coverage.

Why

Phase 7 continues the Python Playwright rollout using the same P0 split as the TypeScript stack.

Verification

• rtk python3 tools/check-scenarios.py --write-matrix && rtk git diff --exit-code docs/scenario-matrix.md → passed.
• cd stacks/python-playwright && .venv/bin/ruff check . && .venv/bin/ruff format --check . && .venv/bin/mypy src tests → passed.
• rtk npx --yes markdownlint-cli2@0.23.0 README.md stacks/python-playwright/README.md docs/scenario-matrix.md → passed.
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest -m smoke --browser chromium --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/p7-2-smoke --junitxml test-results/p7-2-smoke/junit.xml → passed (6 passed).
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest --browser chromium --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/p7-2-chromium --junitxml test-results/p7-2-chromium/junit.xml → passed (12 passed).
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest --browser firefox -m 'not http and not not_ci and not flaky_demo' --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/p7-2-firefox --junitxml test-results/p7-2-firefox/junit.xml → passed (12 passed).
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest --browser webkit -m 'not http and not not_ci and not flaky_demo' --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/p7-2-webkit --junitxml test-results/p7-2-webkit/junit.xml → passed (12 passed).

Out of scope

Remaining Python P0 scenarios and HTTP/resource checks are reserved for P7.3.