refactor: extract Arel scalar comparisons

[kholdrex]

Summary

Refactors the Arel compiler path for scalar comparison filters so the repeated bind and predicate construction lives in one helper. The change keeps generated SQL behavior, bind ordering, and public APIs unchanged while making the compiler easier to maintain after the recent bind-key cleanup.

Changes

• Extracts shared scalar comparison handling for =, !=, <>, >, >=, <, and <= in the Arel compiler path.
• Keeps between, in, like, ilike, existence filters, and unsupported-operator behavior unchanged.
• Adds focused compiler specs for scalar predicate node selection and bind-spec ordering.

Test Plan

• Focused compiler specs passed.
• Full RSpec suite passed.
• RuboCop passed on the changed compiler and spec files.
• Diff whitespace check passed.