fix: omit partial declarations unless exportUnsafe is true

[robsimmons]

There are no kernel checks ensuring that .partial declarations are inhabited, so .partial declarations are as unsafe as .unsafe ones from a kernel-checking perspective.

This change won't break any theorems that safely rely on partial def whatever ..., because the elaborator emits a safe opaque definition for whatever and only tags whatever._unsafe_rec as .partial. (Lean.Replay skips both .unsafe and .partial when importing, so this also matches that behavior.)