feat: add WithPlatformCode for domain-level error identity

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>

CHANGELOG.md

@@ -5,6 +5,22 @@ All notable changes to this module will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this module adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [0.10.0] - 2026-03-25
+
+### Added
+
+- `(*Err).WithPlatformCode(code string) *Err` — chainable builder method to attach a platform-level error code; returns the receiver for chaining
+- `(*Err).PlatformCode() string` — getter that returns the platform code, or an empty string if none was set
+- `platformCode` field in `MarshalJSON` output — serialised as `"platformCode"` and omitted when empty (`omitempty`)
+
+### Design Notes
+
+- Platform codes operate at the domain/system layer and are intentionally decoupled from the transport-level `Code`. `Code` drives HTTP status mapping and gRPC status codes; `PlatformCode` is a stable semantic identifier for the consuming application (e.g. a frontend performing i18n).
+- Platform codes are **optional**. Errors that do not have a user-actionable meaning (500 internal errors, infrastructure failures, authentication rejections) should not carry one.
+- No existing behaviour changed — `Code`, `Error`, `Unwrap`, `WithContext`, `MarshalJSON` (for errors without a platform code) are all backwards-compatible.
+
+[0.10.0]: https://code.nochebuena.dev/go/xerrors/compare/v0.9.0...v0.10.0
+
 ## [0.9.0] - 2026-03-18
 
 ### Added