✨ Support `if (c)` and `if (c[k])` as boolean-only conditions

[rturrado]

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Problem statement

OpenQASM 3 accepts a classical register or a single classical bit as a boolean condition, with no explicit comparator:

if (c) x q[0];
if (c[0]) x q[0];

The debugger's parser (parseClassicConditionExpression in src/common/parsing/CodePreprocessing.cpp)
requires one of the six comparators (==, !=, <, <=, >, >=) and returns std::nullopt for these bare forms,
so the classic-controlled gate fails to parse.

mqt-core's IfElseOperation already models this shape (a single classical control with an implicit non-zero check),
so the simulation backend is ready. Only the debugger front-end needs to catch up.

Surfaced during the follow-up discussion of #168 (PR #456), which added support for the six non-equality comparators.

Proposed solution

Extend parseClassicConditionExpression to accept a bare register reference (c) or a bare bit reference (c[k]) as a valid condition
and evaluate it as "the value is non-zero".
The existing ClassicCondition and applyComparison(..., qc::Neq) machinery should be enough, so no evaluator change is expected.

Tests should cover both the satisfied and unsatisfied cases, the single-bit form,
and one backward-step case to mirror the coverage added in #456.

Out of scope

• Compound boolean expressions with &&, ||, !.
  OpenQASM 3 supports them, but mqt-core's parser does not; that work would start in mqt-core.
• Multi-instruction if bodies, per-statement stepping, register slicing, and other extensions of if.
  Covered by separate follow-up items.