Preferred defaults ("Favorites") for machines, modules & solid fuel

[ApocDev]

Remember a preferred pick for interchangeable choices and apply it to newly created blocks, leaving existing blocks alone unless they are explicitly migrated.

Choices to cover:

• Machine tier, furnace, and mining drill, per machine category.
• Module loadout.
• Solid fuel, per fuel category. Solid fuels stay explicit picks because they leave fuel-specific byproducts (coal burns to ash, nuclear fuel to a used cell) that the rest of the factory has to deal with.
• Fluid fuel: only which fluid to prefer. Fluid fuel is otherwise modelled as a shared energy pool (tracked in Generic fluid-fuel energy commodity (energy:fluid) for fungible fuel demand #25); this issue supplies the fuel that pool's allocator falls back to.

Solid fuel already has the data it needs: items carry fuelCategory and fuelValueJ, machines map to fuel categories through the machineFuelCategories table, and computeBlock currently auto-picks the cheapest fuel in a category. This replaces that auto-pick with a stored preference.

Behaviour:

• New blocks only. Existing blocks keep their current picks; migrating them is an explicit action, never a silent rewrite.
• A preference for something not yet researched, or for an unpicked TURD option, is stored but inert and takes effect once it becomes available. TURD picks are never auto-applied.
• Scope is probably per-project with an optional global fallback, but that needs deciding.

Open questions: where the UI lives (a settings page versus an inline "set as default" control on the block view), and whether the module-loadout default is global or per machine category.

Related: #25.