[framework] render: merge tessellation points (kill false facet edges in previews)

[BenchCAD]

Fixes the "faceted/rough" look of preview renders (maintainer report on #79).

Root cause: Shape.tessellate duplicates vertices along BRep face borders,
so vtkFeatureEdges saw every face boundary as an open edge and drew it — a
ruled loft rendered with a line at every station even where the dihedral angle
is ~1°, and any multi-face surface looked scribbled.

Fix: run vtkCleanPolyData (point merging) on the tessellated mesh before
the normals/edge filters in render_iso. Only true >35° feature edges and real
boundaries draw now. Before/after on stop_bar_tailpiece: station seam lines
vanish; genuine edges (slot rims, bore rims, tab steps) stay crisp.

Preview-quality only — geometry, validate gates, and hashes untouched (the mesh
used for geometry hashing in validate is unchanged; this filter lives in the
render path).

[github-actions]

bench2 validate

set_screw_shaft_collar

  ✓ family.json: keys + base_plane valid
  ✓ pieces: build(6 params) + PARAM_SPEC + check present
  ✓ PARAM_SPEC: 6 params, all entries complete
  ✓ easy: 4/4 seeds sample+check+build+execute clean
  ✓ medium: 4/4 seeds sample+check+build+execute clean
  ✓ hard: 4/4 seeds sample+check+build+execute clean
  ✓ coverage: bore_d reaches all 15 declared values (standard table fully covered)
  ✓ difficulty separation: difficulties produce distinct programs
  ✓ geometry novelty: 7/12 unique shapes (42% duplicate)
  ✓ solids: every instance non-degenerate, 1 solid(s) each
PASS — designs/set_screw_shaft_collar

slotted_din_rail

  ✓ family.json: keys + base_plane valid
  ✓ pieces: build(10 params) + PARAM_SPEC + check present
  ✓ PARAM_SPEC: 11 params, all entries complete
  ✓ easy: 4/4 seeds sample+check+build+execute clean
  ✓ medium: 4/4 seeds sample+check+build+execute clean
  ✓ hard: 4/4 seeds sample+check+build+execute clean
  ✓ difficulty separation: difficulties produce distinct programs
  ✓ geometry novelty: 6/12 unique shapes (50% duplicate)
  ✓ solids: every instance non-degenerate, 1 solid(s) each
PASS — designs/slotted_din_rail

speaker_pole_mount_socket

  ✓ family.json: keys + base_plane valid
  ✓ pieces: build(8 params) + PARAM_SPEC + check present
  ✓ PARAM_SPEC: 8 params, all entries complete
  ✓ easy: 4/4 seeds sample+check+build+execute clean
  ✓ medium: 4/4 seeds sample+check+build+execute clean
  ✓ hard: 4/4 seeds sample+check+build+execute clean
  ✓ coverage: bore_d reaches all 2 declared values (standard table fully covered)
  ✓ coverage: flange_od reaches all 2 declared values (standard table fully covered)
  ✓ coverage: flange_t reaches all 2 declared values (standard table fully covered)
  ✓ coverage: depth reaches all 6 declared values (standard table fully covered)
  ✓ difficulty separation: difficulties produce distinct programs
  ✓ geometry novelty: 9/12 unique shapes (25% duplicate)
  ✓ solids: every instance non-degenerate, 1 solid(s) each
PASS — designs/speaker_pole_mount_socket

Previews: the previews artifact on this run.