Modeling skill: rim fillets after shell, slots along Y, boolean retry caveats (#879)

Corrects the modeling skill: an edge of the face a shell removes is rounded after the shell, a slot along Y is dimensioned horizontally, long shallow arcs measure to about 0.1%, and a boolean retry warning can hide wrong geometry when the volume does not match.

skills/solidcraft-modeling/SKILL.md

@@ -41,7 +41,7 @@ Work the part out as numbers first. Then build it in a few large `batch` calls,
   - Picks must be plain numbers.
   - A pick is resolved once and then follows its edge when parameters change.
 - **Profiles:** extrude takes every region by default. For a ring or a boss with a hole, pass `profiles`.
-- **Order:** fillet the outside edges before you shell, so the shell follows the fillets.
+- **Order:** fillet the outside edges before you shell, so the shell follows the fillets. An edge of the face the shell removes, such as the rim of an open box, is the exception: round it after the shell, because the shell can fail on it.
 
 ## Verify
 - **Each stage:** run `measure {}` (about 1 KB) and compare the volume, bbox and face count with your numbers. A cut or hole that leaves the volume unchanged has missed, even though it reported success.

skills/solidcraft-modeling/reference.md

@@ -29,11 +29,11 @@ These are the shapes of the commands a part build needs, so an agent rarely has
 - **Curves:** `l1`, `c1`, `a1`.
 - **Points:** `p1`, `l1.start`, `l1.end`, `c1.center`, `a1.center`, and the sketch origin `"origin"`.
 - **Rectangles:** `sketch.rectangle.*` also returns `sides {bottom,right,top,left}` and `corners {bottom_left,...}`.
-  - `rectangle.center` adds 2 construction diagonals, so each corner joins 3 lines.
+  - `rectangle.center` adds 2 construction diagonals, so each corner joins 3 lines. Its centre point is `p5`.
   - `sketch.fillet {point}` therefore fails on its corners. Use `{a:"l1", b:"l4", radius}` or the 3D `solid.fillet`.
 - **Slots:** `sketch.slot.overall` and `sketch.slot.center_to_center` create curves `l1, a1, l2, a2` and points `p1`-`p4`, `a1.center`, `a2.center`.
   - The `length` role of `slot.overall` is the overall length. The arc centres are at ±(length/2 - width/2).
-  - After `length` and `width`, 3 dof remain. To remove them, dimension `origin` to `a1.center` horizontally and vertically, and `origin` to `a2.center` vertically.
+  - After `length` and `width`, 3 dof remain. To remove them, dimension `origin` to `a1.center` horizontally and vertically, and `origin` to `a2.center` vertically. For a slot along Y the solver refuses that last dimension as over-constraining; dimension `a2.center` horizontally instead.
   - `sketch.slot.center_point` creates no centre point. Dimension an arc centre instead.
 - **Circles:** `sketch.circle.center` creates `c1` and `c1.center`, and no `p` point.
 - **Dimensions** share the `k` counter with constraints.
@@ -74,7 +74,7 @@ Distances are unsigned. Draw the geometry on the correct side first, and the dim
 | `solid.cylinder` | `radius` or `diameter`, `height` | `base` is the centre of the bottom (default origin), `axis` defaults to `Z`. Expressions work in `base`. Always pass `operation`. |
 | `sketch.slot.overall` / `sketch.slot.center_to_center` | `p0`, `p1` (overall ends / arc centres), `width` | Ids and dof are in the "Ids" section above. |
 | `solid.fillet` / `solid.chamfer` | `edges:[[x,y,z]]`, `radius` / `distance` | The pick snaps to the **nearest** edge, with no distance limit and no error, so check that the face count changed. One pick takes the whole tangent chain: a rounded rim needs 1 point, not 8. |
-| `solid.shell` | `faces:[[x,y,z]]`, `thickness` | A point on each face to remove. Fillet first. |
+| `solid.shell` | `faces:[[x,y,z]]`, `thickness` | A point on each face to remove. Fillet first, except an edge of a removed face (see Order in `SKILL.md`). |
 | `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. |
@@ -122,10 +122,10 @@ After `set_parameter L=120` the volume is 35335.2 mm³, and the holes stay in th
 
 ## Reading `measure`
 
-- **Tolerance:** analytic and measured volumes agree to about 0.001-0.01%, so treat a gap that small as a match.
+- **Tolerance:** analytic and measured volumes agree to about 0.001-0.01%, so treat a gap that small as a match. Long shallow arcs measure worse, about 0.1% (R 1000 mm over 20 degrees).
 - **Face counts:** compare the logical `faces`, not the larger `kernel.faces`.
 - **Reply types:** `set_parameter` returns `recomputed` as a number. A batch reply echoes every step.
-- **Boolean warning:** a feature `warning` such as `Boolean retry: temporary coincident-face separation up to 2.050000e0 mm per face` means the kernel nudged faces that coincide. Examples are a hole floor on another floor, or a join flush on a face. When the volume still matches your number the warning is harmless.
+- **Boolean warning:** a feature `warning` such as `Boolean retry: temporary coincident-face separation up to 2.050000e0 mm per face` means the kernel nudged faces that coincide. Examples are a join flush on a face, such as a cylinder joined onto the floor of a shelled box, or a tapered cut that starts on a face. When the volume still matches your number the warning is harmless. When it does not, the retry may have left the geometry wrong, with no error. Seen: on a box with filleted corners, a shell and a rounded rim, a standoff joined and its pilot hole cut as two cylinders, patterned 2x2, left material in the holes at some standoff heights. A boss with its hole as one ring profile (two concentric circles, extrude the ring) built correctly at every height tried (3 to 20 mm). Another kind, `tool displacement`, moves a tool by about 2e-4 mm.
 
 ## Reply cost (0.8, plate example)