Feature mirrors and patterns: preserve replay and add Flip/Center placement (#1619)

Thanks @Karl-VonRichter.

crates/doc/src/format/pending/feature-pattern-alignment.rs

@@ -0,0 +1,4 @@
+//! Rectangular and circular feature patterns store alignment and reversal, independently for
+//! each rectangular direction. Defaults preserve older positive-direction placements.
+//! Center retains the source and alternates copies on either side, as sketch patterns do.
+//! The last release refuses this pending format and asks for an update instead of losing it.