macOS: bundle only the enchant applespell backend

[jamesbannan]

Summary

Slims the macOS DMG build (build-mac.yml) to bundle only the enchant applespell backend instead of every backend Homebrew ships.

On a clean Mac the aspell backend is dead weight: its word-list data is not bundled, so it registers zero dictionaries while still dragging libaspell into the app via Mach-O dependency analysis. Spell-check on the shipped app is provided entirely by the applespell backend, which delegates to the macOS system spell checker (NSSpellChecker) and needs no bundled dictionary data.

Changes

• Bundle only enchant_applespell.so (drop the for so in .../enchant-2/*.so loop).
• Add a build-time check that enchant_applespell.so exists in the Homebrew prefix (fail fast otherwise).
• Tighten the post-build guard to require enchant_applespell.so specifically instead of "at least one .so".

Validation (arm64, Apple Silicon)

Built a frozen novelWriter.app with this exact enchant logic, then hid Homebrew's enchant backends and aspell data to simulate a clean user Mac:

• enchant-2/ contains only enchant_applespell.so; libaspell is no longer bundled.
• Direct enchant check: providers ['AppleSpell'], 43 dictionaries (en_GB/en_US/en_AU/de_DE/fr_FR, ...), en_GB checks colour OK / wrongg rejected and suggests "language" for "langauge".
• Real app launch (its own runtime hook, no manual env var) logs:

Checking package 'pyenchant': OK
Ready: SpellEnchant
Enchant spell checking for language 'en' loaded

This confirms the shipped .app spell-checks via the macOS system dictionaries with zero bundled dictionary data (fixes the empty-dropdown regression from saga-soft#2705), now without the dead aspell/libaspell weight.

Relates to saga-soft#2748.