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`).

.github/workflows/ci.yml

@@ -778,15 +778,36 @@ jobs:
       - name: Build the C ABI library
         run: cargo build -p wickra-c --release
 
+      - name: Vendored header in sync with the C ABI
+        shell: bash
+        # bindings/go/include/wickra.h is a committed copy of the cbindgen header
+        # (the parent ../c/include is outside the Go module, so it must be
+        # vendored). Fail if it drifts from the source of truth.
+        run: |
+          if ! diff -u bindings/c/include/wickra.h bindings/go/include/wickra.h; then
+            echo "::error::bindings/go/include/wickra.h is stale — copy bindings/c/include/wickra.h over it"
+            exit 1
+          fi
+
       - name: Stage the native library
         shell: bash
+        # Stage into lib/<goos>_<goarch>/ to match the per-platform cgo LDFLAGS.
+        # CI builds the host target, so RUNNER_OS/ARCH give the right directory
+        # (note macos-latest is arm64).
         run: |
-          mkdir -p bindings/go/lib
-          case "$RUNNER_OS" in
-            Linux)   cp target/release/libwickra.so    bindings/go/lib/ ;;
-            macOS)   cp target/release/libwickra.dylib  bindings/go/lib/ ;;
-            Windows) cp target/release/wickra.dll       bindings/go/lib/ ;;
+          case "$RUNNER_ARCH" in
+            X64) arch=amd64 ;;
+            ARM64) arch=arm64 ;;
+            *) echo "::error::unsupported RUNNER_ARCH '$RUNNER_ARCH'"; exit 1 ;;
           esac
+          case "$RUNNER_OS" in
+            Linux)   dir="linux_$arch";   lib=target/release/libwickra.so ;;
+            macOS)   dir="darwin_$arch";  lib=target/release/libwickra.dylib ;;
+            Windows) dir="windows_$arch"; lib=target/release/wickra.dll ;;
+          esac
+          mkdir -p "bindings/go/lib/$dir"
+          cp "$lib" "bindings/go/lib/$dir/"
+          echo "WICKRA_GO_LIBDIR=$PWD/bindings/go/lib/$dir" >> "$GITHUB_ENV"
 
       - name: Go info
         run: go version
@@ -801,9 +822,11 @@ jobs:
 
       - name: Vet and test the Go binding
         shell: bash
-        # On Windows there is no rpath; the loader resolves wickra.dll via PATH.
+        # On Windows there is no rpath; the loader resolves wickra.dll via PATH
+        # (WICKRA_GO_LIBDIR is the per-platform staged lib dir). Linux/macOS use
+        # the rpath baked by the per-platform cgo LDFLAGS.
         run: |
-          export PATH="$PWD/bindings/go/lib:$PATH"
+          export PATH="$WICKRA_GO_LIBDIR:$PATH"
           cd bindings/go
           go vet ./...
           go test ./...
@@ -811,7 +834,7 @@ jobs:
       - name: Build the Go examples
         shell: bash
         run: |
-          export PATH="$PWD/bindings/go/lib:$PATH"
+          export PATH="$WICKRA_GO_LIBDIR:$PATH"
           cd examples/go
           go build ./...
 
@@ -819,7 +842,7 @@ jobs:
       - name: Run the offline Go examples
         shell: bash
         run: |
-          export PATH="$PWD/bindings/go/lib:$PATH"
+          export PATH="$WICKRA_GO_LIBDIR:$PATH"
           cd examples/go
           for d in streaming backtest multi_timeframe parallel_assets \
                    strategy_rsi_mean_reversion strategy_macd_adx strategy_bollinger_squeeze; do