Fix C++ multi-column support for 3+ columns queries

[JasonAtClockwork]

Description of Changes

A user reported a possible issue with ranged queries with multi-column indexes and during my investigation I found it was a deeper problem in how the original work handled situations with more than three columns.

• Reworked exact scans vs range scans to support proper use for multi-column indexes
• Reject invalid use case where the range is not the terminal value
• Fixed an issue with index iterators for buffer resizing on large payloads returned through index scans
• Updated missing documentation for multi-column queries

API and ABI breaking changes

No intended breaking changes.

This change correctly rejects invalid usage such as (exact, Range<>, exact), which was previously allowed but failed host-side.

Expected complexity level and risk

3 - Changes to the internals of IndexIterator to handle iterator buffering is noteworthy but the public API should not be impacted.

Testing

• Built a local throwaway project with a matrix of tests across 1->6 columns and exact vs range
• Added and ran small compile side test suite