Add sorting to add-modifier list

[mcagnion]

Description of the problem being solved:

The "Add Modifier to Item" popup in the Items tab lists candidate mods for every source available on the item — prefix / suffix / implicit tiers, bench/master crafts, essences, veiled mods, crafted mods, influenced mods — in source order. When picking between dozens of candidates the user has to eyeball which one most helps the current build, regardless of source.

This PR adds a Sort dropdown to the popup. The metrics are drawn from data.powerStatList (same source as CompareTab, ItemDBControl, NotableDBControl, SkillsTab, TradeQuery, and RadiusJewelData), so the available sorts stay in sync with every other sort surface in PoB. Selecting a stat recomputes each candidate against the current build via calcsTab:GetMiscCalculator() and orders them by the resulting output value. "Default" restores the original source order. The sort applies to the current Source filter, so switching Source re-applies automatically with cached values.

Steps taken to verify a working solution:

• Ran busted --run=quick --lua=luajit --output=gtest ../spec (full spec tree) and got 208 successes / 0 failures / 0 errors in 32 s.
• Manually verified in the Items tab that:
  • The Sort dropdown is populated with every eligible data.powerStatList entry.
  • Sorting by Life, Effective Hit Pool, Life Regen, Armour, etc., promotes the expected mods on a sample build.
  • The sort respects the current Source filter (e.g., Source = Suffix + Sort = Life Regen lists only suffix mods ordered by life regen gained).
  • Switching between Source types re-applies the current sort.
  • Selecting Default restores the original source order.
  • The cached sortValues table avoids re-computing when the user switches back to a previously-selected stat.

Before — popup without Sort dropdown:

After — sorted by Life:

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After — sorted by Effective Hit Pool:

After — Source = Suffix, sorted by Life Regen: