Events.forEach/subscript and Ready.isReadClosed/isWriteClosed are on the
per-event path of a consumer's event loop. Marking them @inlinable lets
the optimizer inline the per-event dispatch across the module boundary
(restoring intra-module inlining behaviour from before MIO was a separate
package). buffer is widened to @usableFromInline internal to back this.

Pure compile-time hint: no runtime cost, no behaviour change.