Adds an optional PlatformCode field to *Err, decoupled from the
transport-level Code. Code drives HTTP/gRPC status mapping;
PlatformCode is a stable domain identifier for consuming applications
(e.g. a frontend performing i18n) to map errors to localised messages.
Platform codes are optional — errors without a user-actionable meaning
(500s, infrastructure failures, auth rejections) carry none.
Fully backwards-compatible: no existing signatures or JSON output changed
for errors without a platform code.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
