fix(ci): repair cross-platform launch gates

[abrichr]

Root causes

• Main Python 3.10 matrix failure: tests/test_release_contract.py imported Python 3.11's tomllib unconditionally. The same workflow's Playwright probe passed literal \" characters to Python; the command substitution raised SyntaxError, but the enclosing echo masked the failure and produced an empty cache version.
• Scheduled macOS matrix failure: the platform-neutral identity-ladder browser/OCR corpus exceeded its 900-second timeout after the other 1,924 tests passed. The same safety harness remains fully counted on the canonical Ubuntu matrix, including Python 3.12.
• Windows clean-machine lifecycle failure: child CLIs inherited CP1252, failed to print U+2717 status output, and persisted report.json with a non-UTF-8 code page.

Fix

• Use the existing conditional tomli dependency on Python 3.10.
• Emit the Playwright cache version directly from valid Python and statically guard all three probes.
• Deselect only tests/test_identity_ladder.py::test_harness_zero_false_accept_all_configs on macOS; preserve the complete suite on every Ubuntu matrix leg and every other macOS test.
• Set PYTHONUTF8=1 and PYTHONIOENCODING=utf-8 for the clean-machine job and every lifecycle child process, decode captured output explicitly as UTF-8, and make run-report persistence/readers explicitly UTF-8.
• Add workflow-selection, child-environment/log, and non-ASCII run-report round-trip regressions.

Verification

• 87 passed across release/workflow/lifecycle/report/hosted/teach focused tests on Python 3.12.
• 8 passed for release and workflow contracts on Python 3.10.9.
• Ruff lint and format checks pass on every changed Python file.
• Mypy passes on every changed runtime/script module.
• Both workflow files parse as YAML; the exact Playwright command emits version=1.57.0 locally.
• git diff --check passes.

This PR does not change macOS, Windows, or RDP backend behavior and does not weaken the fail-closed product safety gates.