Add cpanfile for non-core dependencies

[tomjn]

What

Adds a cpanfile declaring the non-core Perl modules currently described only in
prose in INSTALL.md:

• FFI::Platypus (required, non-Windows) / Win32, Win32::API,
  Win32::TieRegistry (Windows), gated on $^O
• IO::Socket::SSL (recommended)
• DBD::SQLite (recommended, multi-instance mode)

Why

Makes the dependencies machine-readable, enabling cpanm --installdeps . and use
by CI. Core modules are intentionally omitted, and INSTALL.md remains
authoritative for the preferred system-package install method (the cpanfile
comments point back to it).

Purely additive; no runtime code is touched. Validated that the file evaluates
cleanly and declares the expected modules.

[Yaribz]

SPADS isn't a standard Perl program/module. It's not published on CPAN, it self-updates, its directory structure doesn't follow the standards, and so on. Its dependencies are usually provided directly by system-provided packages. I'm a bit mitigated about this PR, because providing a cpanfile might misleadingly suggest that SPADS integrates seamlessly into the CPAN ecosystem and that it is the preferred way to retrieve dependencies.

I guess a cpanfile would only be useful in the very specific case you mentioned: preferring to use cpanm to fetch/build the dependencies rather than directly using the system-provided prebuilt packages. Could you explain why you would prefer maintaining a cpanfile and use cpanm for CI rather than directly retrieving the system-provided packages in the CI actions (as currently done in the Automated tests GitHub action ?