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>

CLAUDE.md

@@ -68,10 +68,18 @@ Vcpkg triplet is auto-resolved from the host platform by
 Windows, `x64-linux` on Linux, `arm64-osx` on Apple Silicon. See docs/building.md §1
 "Platform matrix" for details.
 
-One-time vcpkg setup:
+vcpkg is bundled as a git submodule at `vcpkg/`, pinned to the commit in `vcpkg.json`'s
+`builtin-baseline`. One-time setup after cloning:
+
+```bash
+git submodule update --init vcpkg
+./vcpkg/bootstrap-vcpkg.sh               # .bat on Windows
+```
+
+To use an external vcpkg checkout instead (e.g. one shared across projects), set
+`VCPKG_ROOT` — it always takes priority over the bundled submodule:
 
 ```bash
-# one-time: git clone https://github.com/microsoft/vcpkg && ./vcpkg/bootstrap-vcpkg.sh  (.bat on Windows)
 export VCPKG_ROOT=/path/to/vcpkg         # works on Linux / macOS / Windows
 ```