Investigated the last deferred enhancement. List(T).append at comptime
fails in two independent layers (both reproduce with plain List(i64);
List works via #run because that evaluates at emit time, after lowering):
1. null comptime allocator — defaultContextValue looks up the
CAllocator->Allocator thunks by name, but they aren't lowered at
scanDecls time. Fixable by forcing getOrCreateThunks before the interp
runs in runComptimeTypeFunc (tried, works for this layer).
2. struct_get through a *T slot_ptr chain (the *List receiver) — the
deep part; comptime pointer/struct/slot resolution, its own session.
Speculative fixes reverted (no end-to-end win without layer 2).
85c1b85f8b