feat(pkg): solve all requested platforms together - #15982

## Summary

- Add a trace-based regression for a lock directory with exactly one
requested platform.
- Assert the exact SAT problem size for a fixed one-package input: two
variables and two clauses.
- Guard against adding cross-platform consistency encoding to the
single-platform path.

This is an independent test-only performance baseline for the
joint-solver work in #15982.

## Tests

- `nix develop -c dune runtest
test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-single-platform-sat-size.t`
- `CI=true nix develop -c dune build @fmt @check`

## Summary

- Add a portable lock-directory regression with distinct extra files
attached to two versions of one package.
- Constrain the solve to the common selected version and assert that its
file is copied into the lock directory.
- Assert that files belonging only to the rejected version are omitted.

This is an independent test-only baseline for per-platform lock
extraction in #15982.

## Tests

- `nix develop -c dune runtest
test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-common-version-extra-files.t`
- `CI=true nix develop -c dune build @fmt @check`

## Summary

- Add a portable lock-directory regression for an unfiltered dependency
alternative whose implementations are available on different platforms.
- Assert that locking preserves the implementation selected for each
platform.
- Build the resulting lock directory on Linux and macOS to cover
consumption as well as the lock summary.

This snapshots the existing per-platform selection invariant before the
joint-solver change in #15982.

## Tests

- `nix develop -c dune runtest
test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-platform-alternative-selection.t`
- `CI=true nix develop -c dune build @fmt @check`

## Summary

- Add a regression documenting that existing language versions currently
allow a portable lock directory to choose different versions of one
package on different platforms.
- Use mutually exclusive Linux and macOS availability filters to make
that behavior explicit.
- Establish the compatibility baseline changed by the joint solver.

This test intentionally documents legacy behavior; #15982 updates its
expectation to the intended all-platform failure when no common version
exists.

## Tests

- `nix develop -c dune runtest
test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-language-version-compatibility.t`
- `CI=true nix develop -c dune build @fmt @check`

## Summary

- Add a portable lock-directory regression for a local package whose
dependency and conflict formulas contain platform filters.
- Request Linux explicitly and assert that `foo.2` remains selectable
there.
- Guard against evaluating local-package formulas in a platform-less
environment before multi-platform solving.

This is an independent test-only baseline for the joint-solver work in
#15982.

## Tests

- `nix develop -c dune runtest
test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-local-platform-constraints.t`
- `CI=true nix develop -c dune build @fmt @check`

## Summary

- Add a portable lock-directory regression for a package that exists in
the repository but is unavailable on Linux.
- Assert that the solver reports `Availability condition not satisfied`
for the requested Linux platform.
- Guard against reconstructing rejection reasons in a platform-less
environment.

This is an independent test-only baseline for the joint-solver work in
#15982.

## Tests

- `nix develop -c dune runtest
test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-platform-rejection-reason.t`
- `CI=true nix develop -c dune build @fmt @check`

## Summary

- Record the current per-platform diagnostic behavior before joint
multi-platform solving changes it.
- Cover a conditional dependency failing on one requested platform and
identical failures on two of four requested platforms.
- Cover a pin whose availability differs by platform, including
alternative selection and the mandatory-pin failure path without
reporting repository versions shadowed by the pin.
- Correct stale prose for non-boolean availability filters, which are
already treated as unavailable without printing a warning.

This is a green preparatory change for #15982; it does not change solver
behavior.

## Checks

- `dune runtest
test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-all-or-nothing.t
test/blackbox-tests/test-cases/pkg/portable-lockdirs/portable-lockdirs-platform-rejection-reason.t
test/blackbox-tests/test-cases/pkg/unavailable-packages.t`