refactor(node): switch oaknode to refcounted value handles, migrate consumers
- all 15 OakNode* handle types become neutral by-value structs
{ctx, addref, release, abi_version}; shared box in
src/node/c_api/nodehandle.h with owns flag (borrowed accessors
return non-owning boxes; graph insertion flips owns off)
- oaktimeline/oaktask/oakrender call sites and their own public
headers migrated to value handles; identity comparisons in
timeline/task now compare native pointers
- regressions green: oaknode 96, oaktimeline 117, oaktask 106,
oakrender 44, oakcommon 193, oakcodec 18, oakaudio 36
Mike-Solar
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