🐛 Register-reference validation is incomplete in the parser

🤖 AI text below 🤖

Problem statement

The parser has three related gaps in how it validates register references and comparison values:

1. Well-formed identifier checking. Neither validateTargets (for qubit targets in gate calls) nor parseBitRegisterRef (for classical bits in if conditions) rejects malformed register names on lexical grounds. Inputs like c2] (dangling ]) or @# (not an identifier at all) slip through the shape check. On the qubit side the eventual definedRegisters lookup rejects them indirectly with an "unknown register" error; on the classical side there is no lookup, so the error surfaces even later in the DD backend.

2. Classical bit reference against declared registers. validateTargets verifies that qubit targets exist in definedRegisters and that the bit index fits within the register size. Nothing analogous runs for the register mentioned in an if condition: if (nonexistent == 1) and if (c[999] == 1) (with creg c[3]) both parse without complaint.

3. expectedValue within the register's representable range. if (c == 999) on creg c[3] (3 bits, values 0 to 7) is impossible to satisfy but the parser accepts it. The comparison always evaluates to false at runtime.

Surfaced during the review of PR #463.

Proposed solution

Add the three missing checks and share them where the same code path applies to qubits and classical bits:

• Extract a small isValidIdentifier helper (letter or underscore followed by letters, digits, or underscores). Use it in validateTargets (before the definedRegisters lookup) and in parseBitRegisterRef (before returning the reference).
• Extend preprocessCode so it also runs an existence + bounds check on the register mentioned in the condition of an if, using the same definedRegisters map already available there. The exact placement (inside parseClassicConditionExpression or a separate validateClassicCondition pass) is an implementation detail.
• Reject expectedValue values that exceed what the referenced register can hold (whole register or single bit).

Out of scope

• Reserved-word checking (qreg, creg, measure, etc.). Separate semantic concern.
• Compound expressions in the condition (out of scope of the parser as a whole today).

Start by tracing preprocessCode and the mentioned entry points validateTargets, parseBitRegisterRef, and parseClassicConditionExpression, using definedRegisters to understand the existing qubit checks. Add identifier, register existence, index-bounds, and expectedValue range validation for both relevant paths. Done means malformed references and impossible comparisons are rejected while the listed out-of-scope cases remain unchanged.