Build the examples during make test

[SeanTAllen]

The examples target is not in .PHONY, and examples/ is a real directory, so make treated the target as already satisfied and never ran the recipe. make test reported success without compiling any example.

Turning the target on surfaced that digest-example no longer compiles: it calls Digest.sha256() outside a try, which stopped being valid when #92 made the Digest constructors partial. This fixes the example in the same change, since enabling the guard is what exposes the breakage.

Closes #72

[SeanTAllen]

Autonomous-mode decision log — PR-specific

Issue #72's premise was false, and this PR fixes more than the Makefile.
The issue says the examples compile by hand. examples/digest-example does not:
it calls Digest.sha256()? and Digest.shake256(64)? outside any try, which
stopped being valid when #92 made the Digest constructors partial. Nobody saw
it because the examples target has never actually run. Options were to ship the
one-line .PHONY fix and let CI go red, split the example fix into its own PR, or
fix it here. I fixed it here: adding examples to .PHONY is what makes the
breakage visible, so merging the guard without the fix means merging a PR whose
whole purpose is to fail.

Cross-cutting decisions that touch every PR (scope of "the tasks", review depth)
are in the summary I gave alongside these PRs.