default_value_ref allocates a DynamicMessage and clones the pool Arc on every get() of an unset message field

[iainmcgin]

default_value_ref in buffa-descriptor/src/reflect/dynamic.rs answers get() on an unset singular message field by building a value on every call:

FieldKind::Singular(SingularKind::Message(midx)) => ValueRef::Message(ReflectCow::Owned(
    alloc::boxed::Box::new(DynamicMessage::new(Arc::clone(pool), midx)),
)),

so each read of an unset message-typed field costs a heap allocation, an Arc clone of the pool (an atomic RMW on a line shared across threads — see the companion issue on the pool's count line), and the matching drop, where the list and map arms of the same function hand out 'static empties for free. A reflective reader that probes optional sub-messages (has-then-get patterns, or readers that treat unset as the empty message per proto3 semantics) pays this per field per message per request.

Options that keep get()'s signature:

• Cache one empty DynamicMessage per MessageIndex in the pool (built lazily, e.g. a OnceLock<Box<DynamicMessage>> slot per message, or a side Vec filled at pool build) and return ValueRef::Message(ReflectCow::Borrowed(&empty)). The empty message's own pool field is the one wrinkle — an empty message never dereferences it for field storage, so it could hold a Weak or the pool could store the empties without a back-reference; whichever fits the existing invariants.
• Or return a dedicated ValueRef::EmptyMessage(MessageIndex)-style variant and let ReflectMessage consumers treat it as the default instance, avoiding the self-referential question altogether at the cost of a new variant.

The first keeps every caller working unchanged.