Kernel rig: the ring cut frame and the up-down join shell

SMLib's inset now keeps only the regions that are cavity, not the one the offsets close where
they overlap (between the two faces of #545's 0.5 floor, round the up-down join's two empty
octants). The frame's core under the recess is hollowed to a sealed void, so the case says two
shells and its closed form, 21044 mm³.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/kernel-cases/shell/545-shell-ring-cut-frame-open-at-an-outer-wall/expected.json

@@ -3,7 +3,12 @@
   "tangency-labs/solidcraft#545"
  ],
  "what": "the frame of examples/regressions/ring_cut_below_recess_thin_floor (a recess from the top, a ring cut from the bottom, a 0.5 mm floor between) shelled 0.5 mm open at the ring cut's outer wall (action matrix (apps/solidcraft-cli/tests/action_matrix.rs): regressions/ring_cut_below_recess_thin_floor, Frame/F6:side:l2 face.shell)",
- "source": "invariants",
- "method": "every wall of the frame is at least 0.5 thick except the 0.5 mm floor, which a 0.5 shell hollows to nothing there: whether that floor's cavity closes is the offset's choice, so no closed form: one closed valid solid is the right answer",
- "body_count": 1
+ "source": "analytic",
+ "method": "every wall of the frame is at least 0.5 thick except the 0.5 mm floor, which a 0.5 shell hollows to nothing: the floor stays solid, and the core under the recess (x, y 16..84, z 0..18) is hollowed to a sealed void, x, y 16.5..83.5, z 0.5..17.5, the floor and the core's walls between it and the open cavity. The cavity is that void, the ring between the squares 0.5..99.5 and 2..98 for z 0.5..18, the ring between 0.5..99.5 and 5.5..94.5 for z 18..29.5, and x 97.5..98, y 2..98, z 0.5..18 behind the open wall: 67^2 17 + (99^2 - 96^2) 17.5 + (99^2 - 89^2) 11.5 + 0.5 96 17.5 = 109010.5. V = 130054.5 - 109010.5 = 21044, the body's outer shell and the void's",
+ "body_count": 1,
+ "shells": 2,
+ "volume_mm3": 21044.0,
+ "tolerance": {
+  "rel": 1e-06
+ }
 }

tests/kernel-cases/shell/KNOWN_FAILING

@@ -4,7 +4,5 @@
 # Baseline: 45f69c0e5 with SMLib 60b1c6a, aarch64 Linux.
 316-shell-open-beside-a-bottom-round-r2-4-as-large-as-the-wall-refused builds a shell where the rim has no thickness instead of refusing it
 316-shell-open-beside-a-bottom-round-r2-as-large-as-the-wall-refused builds a shell where the rim has no thickness instead of refusing it
-545-shell-ring-cut-frame-open-at-an-outer-wall SMLib status 1001
 616-shell-open-a-rib-side-face-of-a-lid SMLib status 1001
 cov-shell-again-open-at-an-inner-wall-of-a-shelled-box SMLib status 1001
-cov-shell-up-down-joined-squares-open-at-a-side-wall SMLib status 1001