Function oncelock-setpub

Set the cell's value if still empty. Returns true if this call performed the initialisation, false if the cell was already set. This is the host-level building block; the spec's .get-or-init (with an initializer quotation) needs the +Executor effect and quotation-dispatch support that hasn't landed yet.

Kept as a free fn for now: making this a method on (OnceLock a) breaks .oncelock-get dispatch during stdlib bootstrap in a way not yet fully understood. WP-M1 revisit when the bare-impl dispatch quirks are fully debugged.