Restructure the Go binding for self-contained distribution (#236)

## Problem
Plain `go get` + `go build` of the Go binding never worked **as a dependency**:
- cgo `CFLAGS` pointed at `${SRCDIR}/../c/include` — the parent dir is **outside** the Go module, so a proxy-fetched module has no header.
- the library under `./lib` was git-ignored and never shipped; the README told users to `cargo build` it from the workspace, which only works inside a clone, not from the read-only module cache.

cgo has no build hook and Go has no registry, so the only way a consumer's `go get`+build can find the lib is for it to be committed **inside the module the consumer pulls**. Per the design decision, that module is a separate **`wickra-go`** repo (keeps this repo free of committed binaries), populated by the release pipeline — this PR is the in-repo restructure that makes that possible.

## Changes
- **Vendor the header** at `bindings/go/include/wickra.h` (committed copy of `bindings/c/include/wickra.h`); `CFLAGS` → `-I${SRCDIR}/include`. A **CI drift check** fails if the copy goes stale.
- **Per-platform libraries**: cgo `LDFLAGS` become per `GOOS`/`GOARCH`, linking `${SRCDIR}/lib/<goos>_<goarch>/`.
- **CI** stages the host library into `lib/<goos>_<goarch>/` (`RUNNER_OS`/`RUNNER_ARCH` — note `macos-latest` is arm64) and exports `WICKRA_GO_LIBDIR` for the Windows PATH; libraries stay git-ignored here.
- **README**: install via the `wickra-go` module + a contributor build section.

## Validation
- Local **windows/amd64**: `gofmt` clean, `go vet`, `go build`, `go test` all green against the staged library + vendored header.
- Linux/macOS arches → the 3-OS `Go on …` CI job.

Follow-up (Stage 2, separate): create `wickra-lib/wickra-go` + a `release.yml` mirror job (source + 6 platform libs → `lib/<goos>_<goarch>/`, commit + tag), and point the docs at the new import path. Part of the self-contained gap (`todo-11`).

bindings/go/wickra.go

@@ -4,16 +4,21 @@
 // Each indicator is an opaque-handle type with a New<Indicator> constructor and
 // Update/Batch/Reset/Close methods. Handles are freed by Close and, as a
 // backstop, by a finalizer; call Close explicitly to release native memory
-// promptly. The binding links against the prebuilt Wickra C ABI library
-// (libwickra.so/.dylib or wickra.dll) staged under ./lib — see the package
-// README for how to provision it.
+// promptly. The binding links against the prebuilt Wickra C ABI library, staged
+// per platform under ./lib/<goos>_<goarch>/, with the C ABI header vendored
+// under ./include. For distribution the libraries are committed alongside the
+// source in the wickra-go module, so `go get` + `go build` works with no extra
+// steps — see the package README.
 package wickra
 
 /*
-#cgo CFLAGS: -I${SRCDIR}/../c/include
-#cgo linux LDFLAGS: -L${SRCDIR}/lib -lwickra -Wl,-rpath,${SRCDIR}/lib
-#cgo darwin LDFLAGS: -L${SRCDIR}/lib -lwickra -Wl,-rpath,${SRCDIR}/lib
-#cgo windows LDFLAGS: -L${SRCDIR}/lib -l:wickra.dll
+#cgo CFLAGS: -I${SRCDIR}/include
+#cgo linux,amd64 LDFLAGS: -L${SRCDIR}/lib/linux_amd64 -lwickra -Wl,-rpath,${SRCDIR}/lib/linux_amd64
+#cgo linux,arm64 LDFLAGS: -L${SRCDIR}/lib/linux_arm64 -lwickra -Wl,-rpath,${SRCDIR}/lib/linux_arm64
+#cgo darwin,amd64 LDFLAGS: -L${SRCDIR}/lib/darwin_amd64 -lwickra -Wl,-rpath,${SRCDIR}/lib/darwin_amd64
+#cgo darwin,arm64 LDFLAGS: -L${SRCDIR}/lib/darwin_arm64 -lwickra -Wl,-rpath,${SRCDIR}/lib/darwin_arm64
+#cgo windows,amd64 LDFLAGS: -L${SRCDIR}/lib/windows_amd64 -l:wickra.dll
+#cgo windows,arm64 LDFLAGS: -L${SRCDIR}/lib/windows_arm64 -l:wickra.dll
 #include "wickra.h"
 */
 import "C"