* feat: add cumulative resource score tracking across partial renders
Add cumulative_render_score and cumulative_assign_score counters to
ResourceLimits that accumulate across reset() calls, with optional
cumulative_render_score_limit and cumulative_assign_score_limit to
cap total work across all partial renders.
Also add a reached? check in BlockBody's render loop so that once a
cumulative limit triggers, the parent template stops processing
further nodes.
Bump version to 5.12.0.
* refactor: move cumulative limit enforcement into reset()
Instead of checking reached? in BlockBody's render loop, enforce
cumulative limits in reset() itself. Since reset() is called before
the begin/rescue MemoryError block in Template#render, the raise
propagates to the parent naturally — no changes to BlockBody needed.
