feat: include platformCode in error responses (v0.10.0)

- Error(w, err) now extracts PlatformCode() from *xerrors.Err and
  includes "platformCode" in the JSON body when set; omitted otherwise
- errorBody updated to accept platformCode as an explicit parameter
- Upgraded code.nochebuena.dev/go/xerrors dependency to v0.10.0
- Added two new tests: platformCode included when set, omitted when absent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -5,6 +5,20 @@ 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
+
+### Changed
+
+- `Error(w http.ResponseWriter, err error)` — now includes `"platformCode"` in the JSON error body when the `*xerrors.Err` carries one; omitted otherwise
+- Upgraded `code.nochebuena.dev/go/xerrors` dependency to `v0.10.0`
+
+### Design Notes
+
+- `platformCode` is the domain-layer error identifier introduced in `xerrors` v0.10.0. It travels through `Error` transparently — no transport-layer logic depends on it.
+- Backwards-compatible: error responses without a platform code are identical to v0.9.0.
+
+[0.10.0]: https://code.nochebuena.dev/go/httputil/compare/v0.9.0...v0.10.0
+
 ## [0.9.0] - 2026-03-18
 
 ### Added