* Add the core execution types, the borrowed context, and the seeded u64 fx hasher
* Fix frame table key aliasing and arena handle offset truncation
* Drop arena entries in reverse allocation order and require Send + Sync payloads
* Draw arena generations from a global counter so foreign handles never upgrade
* Forward serialize through node wrappers and free abandoned frame table reservations
* Make arena generation exhaustion fallible with an unsafe counter rewind
* Guard the shared generation counter in tests and reject oversized batch ranges
* Park the arena during drop glue, guard filled batches, and normalize deserialized sources
* Make the sources fields private and normalize them on every mutation
* Remove the unused CtxSnapshot scope reconstruction
