refactor: extract Arel compiler clause helpers

[kholdrex]

Summary

Extracts the Arel compiler clause-building logic into focused private helpers while preserving generated SQL and bind semantics. This continues the compiler refactor work by reducing the size of the main Arel compilation path without changing behavior.

Changes

• Moves Arel SELECT/projection and DISTINCT handling into dedicated helper methods.
• Moves Arel ORDER BY, GROUP BY, and HAVING application into dedicated helper methods.
• Keeps existing bind ordering and fallback behavior unchanged.

Test plan

• Ruby syntax check passed for the compiler.
• RuboCop passed for the changed compiler file.
• Focused compiler specs passed.
• Full RSpec suite passed.