Feature: Combos as real bundles (component slots, not add-ons) #366

Feature: Combos as real bundles (component slots, not add-ons)

Summary

Model a combo as a product containing selectable component slots (e.g. Lunch Combo $12 → choose 1 Burger, choose 1 Side, choose 1 Drink). Each choice can have a price delta.

Proposed model

combo
  └ combo_group "Main" (choose 1) └ products
  └ combo_group "Side" (choose 1) └ products
  └ combo_group "Drink" (choose 1) └ products

Why not add-ons?

Add-ons snapshot a generic child line on the order item. Combos need each component to behave like a real product: its own inventory, its own kitchen-station routing (burger→Grill, fries→Fryer), and its own tax treatment. Tax categories can differ per component, so this must be carefully scoped (see #282 for tax/tender boundaries).

Sequencing

• Do AFTER product variants (Feature: Real product variants (variant groups/options with SKU, barcode, price delta, optional stock) #357): combo slots build on the same grouping primitives.
• Do NOT fake combos as ordinary add-on groups.

Acceptance criteria

• Combo builder in admin (slots with min/max and eligible products, price deltas).
• POS combo selection UI; resulting bill shows components; KDS routes components to their stations.
• Inventory deducted per component; tax calculated per component category.