feat: #3589846 Add validity stamp infrastructure, save-time integration, and enforcement toggle

Adds a patternkit_block_validity table and ValidityStampWriter service for recording per-language validity state on published block translations. Wires stamp writes into the block save flow, validating against the block's pinned pattern revision with token detection. Adds a render_validation_on_save config toggle (default: enabled) that blocks invalid saves and returns field-level errors, with opt-out for operators managing legacy content.

Depends on update hook 10310 landing first (MR !190 (merged)).

Closes #3589846.