Add optional KYC flow and service-backed account limits

frontend/packages/sxchnge_kyc/README.md

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+# sxchnge_kyc
+
+Shared Flutter identity verification and backend-owned visible limits. Both web
+and future mobile clients consume the same models, providers and service contracts.
+
+- `IdentityProvider` coordinates draft photos, validation errors, submission,
+  refresh and semantic analytics. Inject `IdentityService` and `PhotoService`.
+- `LimitsProvider` fetches `LimitsService` models; it does not calculate caps from
+  a client-selected status. Refresh after status updates. Existing values are
+  marked stale after an error; out-of-order responses cannot replace newer data.
+- `DummyIdentityService` keeps only review metadata in memory. `DummyLimitsService`
+  mimics backend policy with explicitly illustrative EUR fixtures. Neither is
+  production authorization. `review` exists only on the dummy service.
+- `FilePhotoService` opens the platform picker and validates JPEG/PNG content,
+  10 MiB size, minimum 600px per dimension, maximum 40 megapixels and decodability.
+  It performs no face matching, OCR or identity approval. Camera capture is not
+  implemented; users can take a photo separately and select it.
+- Photo models are transient; clients must dispose providers and evict sensitive
+  image cache entries. No disk, browser storage, network or document telemetry is
+  used by these stubs. No guarantee of cryptographic memory erasure is made.
+
+Replace both identity and limits services when connecting a backend. A submit
+adapter must reuse the supplied idempotency key across retries, scope requests to
+the authenticated account, map safe errors and return server-confirmed status.
+Numeric values use minor currency units (currently a two-decimal currency fixture);
+extend the transport with currency precision before supporting other units.
+
+The app's `/identity` route is an explicitly public demo, not an authenticated
+account endpoint. Production requires session/email guards, server authorization,
+private document storage, staff review and support integration. See
+[the UX/measurement contract](../../../docs/ux/kyc.md).
+
+Validation: `flutter pub get`, `flutter analyze`, `flutter test`.
+
+Photo selection uses Flutter's [file_selector](https://pub.dev/packages/file_selector)
+and [image metadata decoding](https://pub.dev/documentation/image/latest/image/Decoder-class.html).
+For native macOS consumers enable the user-selected read-only file entitlement;
+iOS/Android camera permissions are not required for this file-picker-only flow.