✨ Support register-wide Qiskit classical initialization

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Problem Statement

Qiskit classical bits start at zero, whereas an uninitialized OpenQASM 3 bit[n] declaration has undefined contents. The Qiskit serializer therefore currently prepends one explicit store(false) operation per classical bit to preserve Qiskit semantics.

Qiskit can instead store zero into a complete ClassicalRegister, which its OpenQASM 3 exporter writes as:

bit[n] c;
c = 0;

The MQT OpenQASM frontend currently rejects that RHS because whole-register assignments require a bit-register value. This prevents replacing the per-bit initialization with one register-wide operation.

Proposed Solution

Support a well-defined whole-register initialization path for the OpenQASM emitted by Qiskit, then simplify the serializer to initialize each classical register once.

Before implementation, determine whether c = 0 is handled as a standard OpenQASM assignment conversion or as an explicitly documented Qiskit-compatibility form. In either case:

• define width and nonzero-value behavior precisely;
• preserve undefined initialization for OpenQASM declarations that have no assignment;
• cover single- and multi-bit registers, multiple registers, and partially measured registers;
• replace the per-bit serializer loop only after the frontend accepts the exported program;
• coordinate with #2289, which tracks bit-register-to-integer casts in the opposite direction.

Follow-up to #2288 and discussion https://github.com/munich-quantum-toolkit/core/pull/2288#discussion_r3908878547.