fix: align PR #25 with post-#32 main

- refresh-category-map: drop LEAF_FALLBACKS generation (the static fallback
  list was removed in #32 — category fallbacks now come from eBay's runner-up
  suggestions at publish time); update the header docs to match; omit
  unresolved keys from the paste block (a /* TODO */ placeholder between
  commas was a syntax error when pasted verbatim)
- comps: filter and compare comp prices in EBAY_CURRENCY instead of
  hardcoded USD, so a non-US marketplace doesn't get a silently-wrong
  market check
- README: document EBAY_MARKETPLACE_ID / EBAY_CATEGORY_TREE_ID /
  EBAY_CURRENCY as experimental (US is the only tested marketplace, with
  known gaps listed), plus EBAY_STRICT_QUALITY and the per-item-class
  package defaults

Script verified live against the US Taxonomy API: 59/60 keys resolved,
unresolved key correctly flagged, no files modified.