Add clipped Karatsuba Newton reciprocals

[kim-em]

Depends on #9585.

This stacked change closes the remaining generic multiplication slice gap and starts milestone 3:

• replaces the quadratic Karatsuba slice fallback with interval-aware recursive Karatsuba
• computes each of the three recursive subproducts once over the bounding interval of its shifted contributions
• proves the exact MulPlan slice coefficient law for all intervals
• adds plan-driven bounded truncated-series multiplication
• implements Newton reciprocal doubling with both products routed through MulPlan.slice
• proves exact agreement with TSeries.invOfUnit and the multiplicative inverse equation
• adds reciprocal conformance coverage

Verification:

• lake build HexPolyFast
• lake build HexPolyFast.Conformance
• repository-wide lake build was started from a cold cache and stopped after 4,700/10,294 unrelated Mathlib-heavy targets; no failures occurred before stopping