refactor: extract compiler bind spec helpers

[kholdrex]

Summary

Refactors the compiler's bind-spec construction so repeated key and bind metadata handling is centralized in small private helpers. The change preserves existing SQL generation and parameter binding behavior while making the compiler easier to maintain before future compiler work.

Changes

• Adds private helpers for filter bind keys, HAVING bind keys, and bind-spec appends.
• Replaces repeated inline bind-spec hash construction across Arel, string, subquery, policy, and HAVING paths.
• Keeps existing bind ordering, casts, parameter names, and generated query behavior unchanged.

Test Plan

• Focused compiler specs passed.
• Full RSpec suite passed.
• RuboCop passed.
• Independent review found no blocking findings.