Make loader crafting GUI persistent to avoid multiplayer latency delay

[lex]

Backstory

Playing on a slow and unreliable internet connection, we noticed the loader panel next to the crafting menu took seconds to appear, shifting the crafting menu over whenever it finally loaded.

Root cause

The vanilla crafting menu is latency-hidden, so the client renders it immediately. The loader frame however was created inside on_gui_opened and destroyed in on_gui_closed — and script events only run once the open action has made the round trip through the server. On a bad connection that round trip is seconds, so the frame popped in late and re-laid-out the crafting menu when it arrived.

Fix

• The controller-anchored frame is now created once per player (on create, join, force change and research finished) and left in place. Anchored relative frames show and hide with their target GUI, so it now appears in the same frame as the crafting menu with no server round trip. The on_gui_opened redraw is kept as a self-healing refresh.
• The per-entity assembling machine frame keeps its create/destroy cycle and gets its own element name so the two variants no longer collide on lookup.
• New on_register/on_unregister hooks in the QOL feature framework so runtime toggling via set_loaders draws and removes the persistent frames instead of leaking them; on_configuration_changed cleanup covers both frames.
• Fixed a pre-existing nil dereference in draw_loader_frame (player.force was read before the nil check) and nil-guarded the recipe lookups in the click handlers.

Tests

Adds features/redmew_qol_tests.lua for the in-game test runner: frame drawn/removed on feature toggle, frame persists after the crafting menu closes, frame removed when no loader recipe is enabled, one button per enabled recipe, and the machine frame staying separate and destroyed on close. All five pass in-game. Note: run the suite with the redmew-data mod disabled — the loaders feature intentionally goes inert when that mod provides its own loaders.

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[grilledham]

Makes sense. Decent minor qol improvement.