gc: SYMBOL_ACCESSOR_PROPERTIES owner keys are rekeyed but never pruned on owner death #8195

SYMBOL_ACCESSOR_PROPERTIES (crates/perry-runtime/src/symbol/accessors.rs) has a two-part key. The sym_key half IS a strong root (visit_usize_slot, accessors.rs:149). The owner half is rekeyed as metadata only — scan_symbol_accessor_roots_mut (accessors.rs:148) and its budgeted twin scan_symbol_accessor_root_slot (accessors.rs:196), both visit_metadata_usize_slot — so it is rewritten if the owner moved and deliberately not marked.

Nothing prunes it on owner death. prune_dead_symbol_property_owners (symbol.rs:528-550), which gc::dead_owner's fan-out does call, touches only SYMBOL_PROPERTIES and SYMBOL_PROPERTY_ATTRS. The accessor table's only bulk removal, clear_all_symbol_accessor_properties_for_object (accessors.rs:31), is reached solely from the handle-recycle path clear_all_symbol_properties_for_object(handle as usize) (symbol/properties.rs:163) — i.e. never for a plain heap object.

Two consequences: the accessor closures held in those entries leak, and — the #8040 shape — the dead owner address survives to the next cycle's rewrite_raw_addr, where recycled bytes can carry a coincidental GC_FLAG_FORWARDED. #8174 narrowed what can be followed out of such a header; it did not remove the dead key.

Fix: extend prune_dead_symbol_property_owners to the accessor table (it already receives the right predicate and is already registered in DEAD_KEY_PRUNES), with a dead_owner_side_tables.rs case covering both the dead-owner drop and the live-owner inverse.

Declared as open_gap in scripts/gc_rekeyed_key_tables.json (two sites); closing this must delete both entries, or the gate fails on the stale exemptions.

Refs #8174, #8168, #8040.