[config] warn on invalid fail_on / non-bool ml_rules values

[pctablet505]

Unknown fail_on silently falls back to 'high'; ml_rules uses bool() so TOML string 'false' is truthy. Validate and warn.

From the pre-release multi-perspective review (2026-07-03). Follow CONTRIBUTING.md; every change ships a true-positive test + a false-positive guard test.