v0.13.0 - a 32-bit MSVC leg

visual-studio.yml takes an arch input, x64 (the default) or x86.

• arch: x86 configures with -A Win32 against vcpkg's built-in x86-windows
  triplet, the x64-hosted tools cross-compiling. That leg is where MSVC's
  32-bit ABI meets the code: an ILP32 size_t with 8-byte alignment for
  64-bit members, and the STL's portable 128-bit integer class rather than
  the x64 intrinsics. vcpkg_triplet is ignored there, as gcc.yml ignores it
  for x86-linux-gcc.
• A "Require a known target" step rejects anything but x64 or x86.
• The vcpkg cache key carries the arch, so an x86 caller and an x64 caller
  of the same cache_prefix don't evict each other's archives. An existing
  caller's key gains "-x64", so its first run after repinning restores cold.
• A caller passing nothing configures, installs and builds exactly as before.

The README's 32-bit table gains the MSVC x86 row and its stub. Its new
Future directions section records the open design issues: declared support
instead of dropped rungs, ladder freshness, a test for coverage.yml, drift
between callers, and the shared build and documentation.

Callers add one job:

msvc_x86:
uses: rhalbersma/cpp-ci/.github/workflows/visual-studio.yml@ # v0.13.0
with:
arch: x86
tiers: qualification
build_types: Debug

Full changes: #61, #62.