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

Thanks @Karl-VonRichter.

docs/feature-patterns.md

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+# Feature mirrors and patterns
+
+Choose **Mirror**, **Rectangular Pattern** or **Circular Pattern** from the Solid
+Create panel and set **Object Type** to **Features**. Select the source features
+in the timeline or pick their faces in the viewport. Selecting features before
+starting the command fills the Features input automatically.
+
+Feature copies replay the source operations: a patterned hole cuts the target
+solid, and an additive feature adds material. The mirror's New Body / Join
+selector belongs to body copying and is hidden for feature replays.
+
+Select the mirror plane, rectangular directions or circular axis using the
+selection inputs. Click an input to pick into it; click a selected item again to
+remove it, or use its clear button. The preview uses the same commands as OK.
+Cancel leaves the design and undo history unchanged.
+
+Rectangular feature patterns have two direction columns, each with Quantity,
+Spacing, Flip and Center controls. Directions can meet at any angle. A missing
+second direction produces a single row. Direction vectors are saved with the
+feature; editing a picked guide later does not change these saved vectors.
+
+Circular feature patterns have Quantity, Total angle, Flip and Center controls.
+Six instances over 90 degrees use 18-degree steps, including both ends of the
+arc. A full circle divides 360 degrees by the quantity without duplicating the
+source at 360 degrees.
+
+Quantities include the original. Center keeps the original fixed and alternates
+copies on its two sides. For four instances the slots are `0, +1, -1, +2`;
+Flip changes them to `0, -1, +1, -2`. This also chooses the extra side for an even
+centered circular pattern. Rectangular directions have independent controls.
+
+Double-click a saved feature in the timeline to edit it. Mirrors and patterns
+retain their feature sources, including features that created bodies, and
+patterns retain their placement controls and arbitrary direction vectors after
+save/reopen. Source edits recompute the copies. Undo and redo cover the whole edit.
+
+The command forms share sketch patterns' alignment names:
+
+```json
+{"command":"solid.pattern.rectangular","params":{"features":["Hole1"],"dir1":[1,0,0],"count1":4,"spacing1":15,"dir2":[0.6,0.8,0],"count2":2,"spacing2":20,"alignment":"center","alignment2":"center","reversed":true,"reversed2":false}}
+```
+
+```json
+{"command":"solid.pattern.circular","params":{"features":["Hole1"],"axis":{"origin":[40,60,0],"dir":[0,0,1]},"count":6,"angle":"90 deg","alignment":"center","reversed":true}}
+```
+
+Omitted alignments default to `right`, and omitted reversals to `false`, preserving
+older designs. `left` is also accepted by the command API. New placement values
+are stored through the pending document-format migration.