RFC-006: Money as integer minor units

Status: draft proposal for ANP/0.2. Not part of ANP/0.1; nothing here is normative.

#Problem

Offer amounts are JSON numbers ("unit_price": 1140.55). Canonical JSON serializes whatever number arrives deterministically, so hashing is safe, but any implementation that does arithmetic on prices (totals, deltas, threshold checks, the "sum of quantity × unit_price" rule for a missing total_annual) is doing IEEE 754 floating point on money. 0.1 + 0.2 !== 0.3 is enough for two conforming implementations to disagree about whether a stated total_annual matches its line items, and a disputed total in an agreed contract is the worst place to discover it. Several JSON parsers also silently lose precision above 2^53, and the schema permits unit_price up to 10^9 with arbitrary decimal places.

#Proposed change

0.2 offer schema replaces decimal number fields with integer minor units, using the ISO 4217 exponent of the declared currency:

{
  "currency": "USD",
  "line_items": [
    { "description": "CRM Enterprise seats", "quantity": 500,
      "unit": "seat/year", "unit_price_minor": 114055, "currency": "USD" }
  ],
  "total_annual_minor": 57027500
}

unit_price_minor and total_annual_minor are JSON integers (safe-integer range MUST be enforced: absolute values <= 2^53 - 1).
quantity stays a number (quantities are counts and measures, not money) but gains an explicit precision bound: at most 3 fractional digits.
When total_annual_minor is absent, the total is the exact integer sum of quantity × unit_price_minor, computed with the quantity scaled to an integer per its declared precision, so every conforming implementation computes the identical value.
Currencies without minor units (JPY, exponent 0) work unchanged: the minor unit is the unit.

#Compatibility

A field rename plus type change: clean 0.2 material, and mechanical to migrate (unit_price_minor = round(unit_price * 10^exponent)). Keeping the old and new fields side by side in one version invites the exact disagreements this RFC removes, so the change is a hard cut at 0.2.