ossia: reuse value conversion storage

[jcelerier]

Problem

The OSSIA conversion boundary builds temporary standard strings and containers even when the destination already owns compatible storage. Allocator-aware strings are also classified as vectors in some overload sets, and allocator-aware string-key maps are encoded as pair lists instead of OSSIA maps.

Changes

• decode owning allocator-aware strings directly into their existing storage
• preserve active variant string/map alternatives and their allocator/capacity
• reuse native OSSIA list/map destination capacity when source and destination do not overlap
• preserve snapshot semantics for nested source/destination aliasing
• move converted list elements instead of copying them
• encode allocator-aware string-key maps as OSSIA maps
• disambiguate string and optimized numeric-vector output overloads
• register the OSSIA conversion regression target when ossia::ossia is available

Verification

Built and ran tests/test_ossia_values.cpp directly against the score/libossia build configuration:

• 9 test cases
• 97 assertions
• allocator-aware string/map ownership and embedded-NUL round trips
• destination capacity reuse and bounded allocation counts
• std::variant and boost::variant2 active-alternative reuse
• nested native-container aliasing in both containment directions

Also rebuilt score_addon_jk, which exercises these conversions with allocator-aware jq values.