build: bundle vcpkg as a git submodule, pinned to the manifest baseline

Adds vcpkg as a submodule at vcpkg/, pinned to the exact commit vcpkg.json
already declares as builtin-baseline, so the bundled checkout and the
manifest's resolved port versions can never drift apart.

cmake/voicecat-toolchain.cmake, scripts/common.sh, and
clients/apple/scripts/build-xcframework.sh now resolve vcpkg as:
VCPKG_ROOT env var (external checkout) > bundled submodule. Docs updated
to describe the new one-time setup.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

clients/apple/scripts/build-xcframework.sh

@@ -54,7 +54,8 @@ done
 # ── Resolve VCPKG_ROOT ───────────────────────────────────────────────────────────
 # The apple-dev CMake cache records the vcpkg root it was configured with (Z_VCPKG_ROOT_DIR);
 # reuse that so a developer who already configured `cmake --preset dev` doesn't need VCPKG_ROOT
-# in their shell env to run this script.
+# in their shell env to run this script. Falls back to the bundled submodule at
+# <repo-root>/vcpkg if neither the env var nor the cache resolve it.
 if [[ -z "${VCPKG_ROOT:-}" ]]; then
     cache="$REPO_ROOT/build/apple-dev/CMakeCache.txt"
     if [[ -f "$cache" ]]; then
@@ -64,9 +65,16 @@ if [[ -z "${VCPKG_ROOT:-}" ]]; then
         fi
     fi
 fi
+if [[ -z "${VCPKG_ROOT:-}" ]]; then
+    bundled="$REPO_ROOT/vcpkg"
+    if [[ -f "$bundled/scripts/buildsystems/vcpkg.cmake" ]]; then
+        export VCPKG_ROOT="$bundled"
+    fi
+fi
 if [[ -z "${VCPKG_ROOT:-}" || ! -d "$VCPKG_ROOT" ]]; then
     echo "error: VCPKG_ROOT is not set or does not exist." >&2
-    echo "       bootstrap vcpkg (https://vcpkg.io) then:  export VCPKG_ROOT=/path/to/vcpkg" >&2
+    echo "       either init the bundled submodule:  git submodule update --init vcpkg" >&2
+    echo "       or point at an external checkout:   export VCPKG_ROOT=/path/to/vcpkg" >&2
     exit 1
 fi
 echo "[build-xcframework] VCPKG_ROOT=$VCPKG_ROOT"