Regional/PPP pricing + multi-currency GA

Why / Context

PRD §7.2E / §8.6: regional/PPP-adjusted pricing is the mechanism for international expansion (Udemy's existing playbook). Combined with Dodo as Merchant of Record (issue #1), it opens non-India revenue without tax-compliance drag. Schema was built multi-currency from day one precisely so this is activation, not migration.

Schema already in place — NO migration required

• Price.currency + Price.region (nullable = default) on every sellable, including plan interval prices
• Order.currency + BigInt minor units everywhere; Cart.currency
• SubscriptionPlan.providerRefs holds per-currency provider plan ids
• PaymentProviderKind.DODO routing (issue #1)

Implementation plan

1. Region model: define pricing regions (e.g., IN, US, EU, SEA, LATAM, ROW) + geo-detection service (IP header on Netlify → region cookie, user-overridable currency switcher).
2. Pricing engine extension (src/lib/commerce/pricing.ts): resolution order = (currency, region) → (currency, null) → default currency conversion fallback DISABLED (only explicit prices sell — no FX surprises).
3. Studio pricing UI: per-region price rows for courses/projects (bulk "suggest from PPP index" helper using a maintained multiplier table in PlatformConfig); platform sets plan prices per region.
4. Cart/checkout: single-currency cart enforcement (mixed-currency add prompts a switch), provider routing INR→Razorpay / else→Dodo.
5. Subscriptions: create per-currency provider plans; store in providerRefs; upgrade/downgrade stays within currency.
6. Anti-arbitrage: region locked at purchase to detected region unless override + revalidation; VPN-shopping policy documented (soft enforcement: region-mismatch flag on orders).
7. Display: localized currency formatting (Intl), tax-inclusive display per region policy, compareAt strikethroughs per region.
8. Reporting: ledger and payout views group by currency (accounts are already per-currency); FX-normalized revenue reporting for admin (rates table, reporting-only).

Acceptance criteria

• US visitor sees USD prices where defined; items without USD prices show as unavailable-in-region (not FX-converted)
• Checkout in USD routes to Dodo, INR to Razorpay; fulfillment identical
• Region price rows manageable in studio; PPP suggester fills drafts
• Mixed-currency cart is impossible; switcher migrates the cart
• Ledger accounts split by currency reconcile per provider

Dependencies / sequencing

• Requires issue #1 (Dodo MoR). Enhances plans/checkout from MVP M7/M8

Start with src/lib/commerce/pricing.ts, then trace the existing Price, Order, Cart, and SubscriptionPlan providerRefs paths before reading the studio, checkout, and reporting entry points. The work is done when regional prices, currency-specific routing and subscriptions, mixed-currency cart handling, PPP drafts, and the listed ledger and display acceptance criteria are covered without FX fallback.