solid.mirror: a plane's {origin} takes length expressions and follows them (#1492) (#1501)

Thanks @erkexzcx.

skills/solidcraft-modeling/SKILL.md

@@ -36,7 +36,7 @@ Work the part out as numbers first. Then build it in a few large `batch` calls,
   - expression fields such as `solid.cylinder base:["w/2-6","d/2-6","wall"]`
 
   A plain number never follows a parameter.
-- **Repeats:** `solid.pattern.rectangular` takes expression spacing; `solid.mirror` takes `XY`/`XZ`/`YZ`, a construction plane or `{origin, normal}`. A mirror or pattern feature cannot be mirrored or patterned again.
+- **Repeats:** `solid.pattern.rectangular` takes expression spacing; `solid.mirror` takes `XY`/`XZ`/`YZ`, a construction plane or `{origin, normal}` (origin coordinates may be length expressions such as `["half", 0, 0]`). A mirror or pattern feature cannot be mirrored or patterned again.
 - **Edge and face picks** snap to the nearest edge or face. There is no distance limit and no error, so compute the exact midpoint from your geometry (the vertical corner edge of a centred box is `[w/2, d/2, h/2]`).
   - Picks must be plain numbers.
   - A pick is resolved once and then follows its edge when parameters change.

skills/solidcraft-modeling/reference.md

@@ -78,7 +78,7 @@ Distances are unsigned. Draw the geometry on the correct side first, and the dim
 | `solid.hole` | `position:[x,y,z]` on a face + `diameter`, or `sketch` (+ `points`) | `clearance:"M3"`. `depth` defaults to through all. `type` is one of simple/drilled/counterbore/countersink. A `countersink` needs `cs_diameter` and `cs_angle`; a `counterbore` needs `cb_diameter` and `cb_depth`. `position` is a fixed point. |
 | `solid.pattern.rectangular` | `features`, `dir1`, `count1`, `spacing1` (+ `dir2`, `count2`, `spacing2`) | Spacing takes expressions and follows parameters. |
 | `solid.pattern.circular` | `features`, `axis` (`"Z"` or `{origin, dir}`), `count` | `angle` defaults to 360 deg. A full pattern keeps the COM on the axis, so check its radius with a top-view screenshot. |
-| `solid.mirror` | `features:[names]`, `plane` (`XY`/`XZ`/`YZ`, a construction plane, or `{origin, normal}`) | Follows parameters. Mirror and pattern features cannot be mirrored or patterned again. |
+| `solid.mirror` | `features:[names]`, `plane` (`XY`/`XZ`/`YZ`, a construction plane, or `{origin, normal}` whose origin may use length expressions) | Follows parameters. Mirror and pattern features cannot be mirrored or patterned again. |
 
 - **Names:** name every feature (`"name":"Standoff"`) and keep names unique across sketches and features. `timeline.roll_to {"feature":"Plate"}` picks the first match, which can be the sketch.
 - **Counts as parameters:** `parameters.add {"name":"n","expression":"6","unit":""}` creates a unitless parameter, and `"count":"n"` uses it in a pattern.