fix: bump transparency in `isDefEqProj` for class projections

When a class field is marked `@[reducible]`, `isDefEqDelta` unfolds it
to `.proj` form. `isDefEqProj` must then compare the struct arguments,
but the projection's parameter is instance-implicit. Without bumping to
`.instances`, instance definitions (which are `[implicit_reducible]`)
cannot be unfolded, causing `eq_self` to fail. This mirrors the
transparency bump that `isDefEqArgs` applies for instance-implicit
parameters.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>