Harden mruby value lifecycle

[jackalcooper]

What changed

• add isolated VM and rooted Value resources
• retain the parent VM from child values and defer VM destruction until children close
• use mruby GC register/unregister and arena discipline
• contain Ruby exceptions with mrb_protect_error
• stress explicit close and arbitrary BEAM GC ordering

Why

mruby values belong to exactly one mrb_state; the resource graph now makes that ownership explicit and prevents use-after-free regardless of parent/child collection order.

Validation

• MIX_ENV=test mix compile --force --warnings-as-errors
• mix credo --strict
• mix test --force (6 tests)

Stacked on #93.