refactor(domain): tighten the object calisthenics story - #7

- CartItems: a first-class collection absorbing the find/map/filter/reduce that used to
  be duplicated three times in Cart, replacing getItemsCopy()
- Cart now takes item ids (increaseQuantity, decreaseQuantity, removeItem), so the UI no
  longer pulls a Product back out of a CartItem just to hand it back to Cart. Added
  remainingForFreeShipping(), the Money arithmetic that used to live in JSX
- dropped CartItem.getProduct() (the source of that Demeter chain) and the dead
  getPrice(); renamed the rest to id()/name()/image()/quantity() to match the rest of
  the domain's naming
- quantity clamping moved off CartItem onto Quantity (clampBetween, isAtLeast, isAtMost),
  Money got isAtLeast/isZero so policies stop comparing raw .toAmount() values
- Skill's SkillValue and Product's KnowledgeProfile were the same concept with two
  vocabularies (intermediate vs adventurer) bridged by a ProductCardLogic shim. Unified
  on one type, deleted the shim
- GoldSilverCopperFormatter split into GoldSilverCopperAmount (conversion) and
  GoldSilverCopperFormatter (presentation), both got the specs they never had
- Stats.asObject() dissolved the value object into a plain record the UI had to
  re-derive labels from (and rendered "ManaBoost: 0" rows). Stats.list() now returns
  labeled, non-zero entries directly
- a product's lore link and video never actually varied by knowledge level, only the
  data entry did (same two URLs repeated three times per product). Moved ExtraResources
  onto Product itself
- data/products.ts is now a ProductCatalog built from named blueprints instead of a bare
  array of five-deep positional constructor calls
- moved the three WelcomeSurvey storage repositories out of a UI component folder into
  domain/welcomeSurvey/infrastructure/
- shared test fixtures (productFixtures.ts) replace the ~90 lines of product
  construction duplicated across Cart.spec.ts and CartItem.spec.ts

Cart went from 90 lines to 58, CartItem from 75 to 51: close to the 50-line rule, not
exactly on it, and a first-class collection exists now where none did before.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014BcGPRc3sgUJYKihGRXh5g