Define intersection result extraction before the first update

[tisonkun]

ThetaIntersection::to_sketch and TupleIntersection::to_sketch currently panic when called before the first update. Both operators expose has_result, and tests explicitly cover the panic, but absence of an intersection state is part of the public state machine and deserves a deliberate API contract.

We should decide whether result extraction should:

• return Option when no result is a normal state;
• return Result when calling before update is considered recoverable misuse; or
• retain the panic, with a clear invariant and local Panics documentation.

Points to settle:

• Apply the same contract to Theta and Tuple intersections.
• Decide whether has_result remains useful after result extraction becomes typed.
• Define the mathematical meaning of zero updates; the identity for intersection cannot be represented as a finite sketch.
• Consider reset if it is added to the operators later.
• Check Java and C++ precedent, while keeping the Rust API misuse-resistant.
• Document migration if the return type changes before 1.0.

This issue intentionally records the design question rather than prescribing Option or Result immediately.