docs: credit Richard Gooding in syncmap.go file header

[millerjp]

Summary

The majority of `syncmap.go` is Richard Gooding's code from upstream `rgooding/go-syncmap`. The previous per-file header `Copyright 2026 AxonOps Limited.` alone implied AxonOps wrote the whole file. This PR switches to the standard fork attribution pattern.

New header

```go
// Copyright 2023 Richard Gooding
// Modifications Copyright 2026 AxonOps Limited
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
// This file is a fork of github.com/rgooding/go-syncmap; see NOTICE
// for the upstream attribution.
```

Scope

Only `syncmap.go` needed this correction. The other `.go` files are AxonOps-original work:

• `syncmap_test.go` — full rewrite in test: rewrite unit tests to external package with testify/goleak; coverage ≥95% #10 (external package, testify/goleak); upstream smoke test was deleted, zero code overlap
• `doc.go`, `example_test.go`, `syncmap_bench_test.go`, `documentation_test.go`, `tests/bdd/**` — all AxonOps-original, no upstream provenance
• CI workflows, Makefile, scripts — AxonOps-original

So their existing AxonOps-only headers remain accurate.

Validation

• `make check` — green, coverage 100%.
• `apache-header-guard` — matches (header still contains "Licensed under the Apache License, Version 2.0").
• `make llms-full-check` — clean (file headers aren't in go doc output, so no regeneration needed).