[super! 4/6] Reach super metadata by conversion

[mingxwa]

meta_storage looked a meta up with get<M>(), which resolved to a static_cast over a flat set of base classes. That works only while every meta is a direct base of the storage, and cannot reach a meta held inside an aggregate.

Replace it with proxy_meta, whose contained metas are reached through a conversion operator. A meta is contained if the metadata converts to it without throwing, so an aggregate that itself converts to a meta makes that meta reachable too. unique_types_t reduces the meta list so that each entry is contained by exactly one entry, namely itself, which is the property that keeps the conversion unambiguous: a duplicate collapses onto its leftmost occurrence, and a meta subsumed by an aggregate is replaced by that aggregate in place.

Both storages now expose the metadata through operator*, so choosing between them is independent of how a meta is looked up. static_meta_storage holds a single meta and is chosen by size rather than by shape, which separates the metadata layout from the decision to hold it out of line. Under pointer authentication, assigning one storage from another signs a raw pointer, which meta_ptr now supports.