Track shared potion cooldown #7

Show one icon for the shared potion cooldown, without listing potion item IDs.

Why not item IDs

• Using one potion puts every potion in the bags on cooldown; one icon per potion duplicates the timer.
• Potion IDs change every patch/expansion; a list in data/ needs constant upkeep.

Current state

• ProcessCooldownData already has a data.itemId branch (GetItemInfo, C_Container.GetItemCooldown), and button_proto OnEnter tooltips handle itemId.
• The branch is unreachable: core/data.lua has no item constructor, and buildCooldowns drops rows without slotId or a known spellId.
• Events suffice: on Retail and MoP, SPELL_UPDATE_COOLDOWN fires on every item cooldown change (Experiment 10, docs/secret-values-options.md). Item rows have no spellId, so they are always re-checked.

Proposal

One "potion" row with no ID that resolves to whichever potion is in the bags:

• Match by item class: C_Item.GetItemInfoInstant(itemID) → classID, subClassID; potion = Enum.ItemClass.Consumable (0) / Enum.ItemConsumableSubclass.Potion (1). Both exist in the generated docs of every client.
• Scan bags (C_Container.GetContainerNumSlots, C_Container.GetContainerItemID) only on BAG_UPDATE_DELAYED, never on cooldown events. Cache the ID.
• Keep the last ID after the last potion is drunk so the running cooldown still shows.
• Read the cooldown through the existing itemId branch.

Tasks

• Experiments (/filger exp10 <potionID>, prints enabled):
  • MoP: drink in combat, leave combat. Is enabled 0 in combat while start follows GetTime()? Does the cooldown start when combat ends?
  • Era and TBC: same test.
  • Retail: do healing and combat potions share a cooldown? Healthstones? (unverified)
  • Era: are potions classID 0 / subClassID 1, or Generic (0)? (unverified; /run print(C_Item.GetItemInfoInstant(<id>)))
• Data: data.CreatePotionCooldown(enabled) in core/data.lua, one row in data/shared/cooldowns.lua gear, accepted by buildCooldowns.
• Resolve: bag scan setting the row's itemId, at login and on BAG_UPDATE_DELAYED, registered only by the cooldown element.
• Classic in combat: if experiments confirm the pending cooldown, keep the icon hidden while enabled is 0 (FilterCooldown already requires data.enabled).

Open questions

• With potions from different cooldown groups in the bags, which one wins? Unknown until the experiments.