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>

scripts/common.sh

@@ -79,10 +79,10 @@ vc_require_windows() {
 }
 
 # ── VCPKG_ROOT ──────────────────────────────────────────────────────────────────
-# Honor the env var, else try to read it from an existing CMake cache
-# (Z_VCPKG_ROOT_DIR) so a developer who already configured a preset doesn't need
-# VCPKG_ROOT in their shell env. $1 = the preset whose cache to probe as a
-# fallback (e.g. "dev" or "apple-dev").
+# Resolution order: the env var, else an existing CMake cache (Z_VCPKG_ROOT_DIR) so a
+# developer who already configured a preset doesn't need VCPKG_ROOT in their shell env,
+# else the bundled submodule at <repo-root>/vcpkg. $1 = the preset whose cache to probe
+# as a fallback (e.g. "dev" or "apple-dev").
 vc_resolve_vcpkg_root() {
     local fallback_preset="${1:-dev}"
     if [[ -z "${VCPKG_ROOT:-}" ]]; then
@@ -95,9 +95,17 @@ vc_resolve_vcpkg_root() {
             fi
         fi
     fi
+    if [[ -z "${VCPKG_ROOT:-}" ]]; then
+        local bundled="$VC_REPO_ROOT/vcpkg"
+        if [[ -f "$bundled/scripts/buildsystems/vcpkg.cmake" ]]; then
+            export VCPKG_ROOT="$bundled"
+        fi
+    fi
     if [[ -z "${VCPKG_ROOT:-}" || ! -d "$VCPKG_ROOT" ]]; then
         vc_die "VCPKG_ROOT is not set or does not exist.
-       bootstrap vcpkg (https://vcpkg.io) then:
+       either init the bundled submodule:
+         git submodule update --init vcpkg
+       or point at an external vcpkg checkout:
          export VCPKG_ROOT=/path/to/vcpkg"
     fi
     vc_log "VCPKG_ROOT=$VCPKG_ROOT"