Give the MSIX its own numeric version - #5

Every pull request has been failing the Windows job at makeappx, and
never for a reason to do with the change under review:
  error C00CE169: The attribute 'Version' with value '1.0.0-pr4.0'
  failed to parse -- violates pattern constraint
The manifest was built from the DISPLAY version, which is only ever
clean on a tag. On anything else it carries a suffix -- "1.0.0-pr4" for
a PR, "1.0.0-dev" locally -- and the MSIX schema admits four
dot-separated integers and nothing else. A tagged release packed fine,
so this stayed invisible until PRs started building Windows.
MSIX_VERSION is now derived rather than borrowed: the project's numeric
version with any pre-release suffix stripped, then the commit count. The
commit count is there for the same reason CFBundleVersion uses it, and
the reasoning is already written down in CMakeLists -- Windows compares
these numbers to decide what counts as an upgrade, and two builds of
1.0.0 that both claimed 1.0.0.0 would be indistinguishable to it.
Checked against the schema's own regex, which also turned up a case that
had not bitten yet: a pre-release TAG like v1.1.0-beta.2 would have
failed to pack in exactly the same way.
The substitution step now fails if any placeholder survives it, since
the previous failure was one substitution producing something invalid
rather than not happening at all.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>