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/README.md

@@ -16,24 +16,38 @@ cgo and exposes all 514 streaming-first indicators as idiomatic types.
 
 ## Install
 
+Use the published **`wickra-go`** module, which bundles the prebuilt C ABI
+library for every platform, so `go get` + `go build` works with no extra steps
+(a C compiler is still required, as the binding uses cgo):
+
 ```bash
-go get github.com/wickra-lib/wickra/bindings/go
+go get github.com/wickra-lib/wickra-go
 ```
 
-The binding uses cgo, so a C compiler is required, and it links against the
-prebuilt Wickra C ABI library. Build that library from the workspace and stage
-it under this package's `lib/` directory:
+```go
+import wickra "github.com/wickra-lib/wickra-go"
+```
+
+`wickra-go` is generated from this directory by the release pipeline: it mirrors
+the Go sources, the vendored C ABI header (`include/wickra.h`) and the prebuilt
+libraries under `lib/<goos>_<goarch>/`. On Linux/macOS the library path is baked
+in via rpath; on Windows the DLL must be discoverable at run time (next to the
+executable or on `PATH`).
+
+### Building from this repository (contributors)
+
+This `bindings/go` directory is the development source. To build it directly,
+compile the C ABI and stage the library into the per-platform directory cgo
+links against:
 
 ```bash
 cargo build -p wickra-c --release
-cp target/release/libwickra.so    bindings/go/lib/   # Linux
-cp target/release/libwickra.dylib bindings/go/lib/   # macOS
-cp target/release/wickra.dll      bindings/go/lib/   # Windows (also on PATH at run time)
+mkdir -p bindings/go/lib/linux_amd64                 # match your GOOS_GOARCH
+cp target/release/libwickra.so    bindings/go/lib/linux_amd64/    # Linux
+cp target/release/libwickra.dylib bindings/go/lib/darwin_arm64/   # macOS (arm64)
+cp target/release/wickra.dll      bindings/go/lib/windows_amd64/  # Windows
 ```
 
-On Linux and macOS the library path is baked in via rpath; on Windows the DLL
-must be discoverable at run time (next to the executable or on `PATH`).
-
 ## Quick start
 
 ```go

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"