Reconcile swap-to-new-token design doc against the as-built code

Update the design doc to match what shipped: crypto amount as the single
source of truth in both display modes (§2.5), the fail-closed destination
verdict recovery (§3.9), the shared fontScale module (§3.3), the new
useSwapDestinationScan/fontScale test suites (§3.12), and corrected
facts/line-refs (priceValue→priceValueUsd, line count, SlideupModal refs,
MIN_TRENDING_VOLUME7D location, CONFIRM_AMOUNT step).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: CassioMG