Stop parsing unknown value tokens

[iDavi]

Summary

• terminate parsing when a value begins with an unsupported character

Root cause

ParseValue left m_Index unchanged for unknown leading characters. Object and array loops could re-enter at the same position indefinitely.

Validation

• inspected control flow for {x}, [x], and {"a":x}; each now reaches end-of-input deterministically
• no VBA runtime or test suite is available in this environment

Closes #6