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Irakli Kourtis 93a2922c5e Display plans and the model cache for large STEP assemblies (#2814 ) (#4021 )

* STEP import: mesh band faces that wrap around a closed surface correctly

Fixes two related bugs that caused SolidCraft to fail to mesh a face wrapping
all the way around a closed, non-periodic surface (a revolved ridge band, or a
tube-shaped B-spline surface): a missing face, a folded/self-overlapping
triangulation, or a hole in the rendered body.

1. Ring edges that run slightly off their surface. STEP files write a surface
and the curves bounding it to the file's own declared uncertainty, so a
ring edge can legitimately sit a few micrometres off the surface it
bounds. The code finding where a ring edge crosses the planned split line
compared against the edge's raw (off-surface) point using a hardcoded
tolerance far stricter than the file's own precision, rejecting a valid
cut. Now matches against the point on the surface directly below the ring
crossing, respecting the file's declared tolerance.

2. Closed B-spline surfaces, their slits and their slivers. truck only treats
the rotation of a revolved surface as periodic; a face on a B-spline
surface whose control net closes on itself (a tube) wasn't recognized as
wrapping around at all. This produced a folded face, a hole (a boundary
loop landing a full period away from where it should cut), and one
near-zero-width sliver face costing the entire body its tessellation
plan. Adds periodicity detection for closed B-spline surfaces, period-
aware hole-loop alignment, dropping of zero-width slit boundaries, and
isolating a too-thin-to-triangulate face so it no longer takes the rest
of the body down with it. The GPU shader's periodic wrap mirrors the CPU
math so the two don't drift apart.

Regression tests: band_faces_split_when_ring_edges_run_off_their_surface,
band_face_with_a_zero_width_slit_is_still_meshed,
a_sliver_face_too_thin_to_triangulate_does_not_cost_the_body_its_plan.
Ablation-tested: disabling either mechanism in (2) reproduces its target
test's original failure message while leaving the other test unaffected.

* STEP import: mesh the band's split faces, not just split them

jyviko asked whether a band wall missing from the interactive app's render
was a shading issue or a genuinely missing face. It was missing: the ring-
edge split fix made the band's topology correct, but plan_face still
produced zero triangles for the two new sub-faces.

Two causes, both in plan_face (crates/kernel/src/tess/mod.rs):

1. The tolerance for how far an imported boundary may lie off its surface
was tied to the requested mesh's chord tolerance instead of to the
body's own size. A finer display tolerance could then reject a boundary
a coarser one had just accepted, so the face would drop out exactly when
zooming in on it.

2. Near a sharp corner of this flat-panelled band, several boundary points
that are legitimately a few micrometres apart (within the STEP file's
own declared precision) all projected onto the same surface point. The
boundary loop then repeated a point in (u, v), which a triangulator
can't follow, so the face's plan failed outright. One face without a
plan costs the whole body its plan, and the fallback mesher can't
handle a closed B-spline surface either, leaving both halves of the
split band with no triangles at all: present in the topology, invisible
in every renderer, and absent from the body's volume.

Fix: scale the off-surface tolerance to the body's own extent (new
Rules::extent), and collapse a run of boundary nodes that land on the same
surface point to one (drop_collapsed) before triangulating.

Regression test additions to band_faces_split_when_ring_edges_run_off_their_surface:
the body's volume now matches the analytic pentagon prism (was a third of
it - only the top cap - before this fix); display_plan leaves no face
without triangles from coarse to fine tolerance; and the actual tolerance
and facet angle the viewport draws with gives the split band its full
lateral area on both halves.

Confirmed on review_band_big.step, the fixture behind the review screenshots:
volume's relative error against the analytic pentagon prism drops from
1.4e-4 to 4.1e-6, and the band wall now occludes the floor grid behind it
in a snapshot the same way the top and bottom caps always did.

Full kernel suite: 739 tests, 0 failed.

* STEP import: hold an imported boundary to the file's own tolerance, not the part's size

jyviko: the off-surface acceptance added in the last commit
(rules.extent * OFF_SURFACE) scaled with the part, so a 2 m part
silently accepted a boundary 2 mm off its surface while a 20 mm copy
of the same part accepted only 0.02 mm. A STEP file's own declared
uncertainty does not grow with the part, so the bound shouldn't either.

Body gains boundary_tol: Option<f64>, Some(file's declared tolerance)
for a body read from STEP (crate::step_in::Units::tol, already used
by the ring-split fix in split.rs), None for anything this kernel
built. It's set before the import's own unmeshed-face check runs, and
carried over by the two face-split operations that re-cut an imported
surface (splitface.rs, outline.rs), since the file's uncertainty still
describes those boundaries afterward.

plan_face's accept now reads: the file's own tolerance when the body
declares one, or the old chord-tolerance/extent bound otherwise. The
old bound's tol * 10.0 term had to go too, not just the extent term:
at a 2 m part's typical display tolerance that term alone is ~20 mm,
nearly ten times the extent bound jyviko objected to, so keeping it
would have left the exact 2 m/2 mm example still silently accepted.

Two new tests in step_in/tests.rs:
- band_faces_split_when_a_scaled_copy_runs_off_its_surface: the same
band fixture at a realistic 2 m scale with the same absolute slop
and declared uncertainty as the small one, meshes the same way at
every chord tolerance from coarse to fine.
- off_surface_budget_follows_the_file_not_the_part_size: the actual
regression. The same off-surface error is accepted or refused the
same way whether the part is 20 mm or 2 m; a 2 mm error against a
1 µm declared uncertainty is refused at both sizes now (accepted at
2 m under the old bound, refused at 20 mm - that asymmetry was the
bug). Checked at both a coarse and a fine display tolerance.

Known gap, not fixed here: a STEP-imported body saved into a
.solidcraft design and reopened loses boundary_tol (native.rs's
decoder has no field to read it from) and falls back to the old
extent-based bound on reload. Today's design format has no concept of
this value at all; persisting it is a real format change, out of
scope for this fix.

Full kernel suite: 776 tests, 0 failed. clippy -D warnings clean.

* Display plans for STEP threads and pointed ends (#3847)

Three faces cost a threaded or pointed STEP part its whole display plan, so it
was drawn from the reference mesh: cracked, faceted, and never refined up close.

- A loop through a pole (a cone's apex on the boundary) seemed to wind around
the surface: the jump at the apex, unwrapped to the nearest copy, was
arbitrary. The loop is turned to end where it leaves the pole, so the step
that closes it is an ordinary one (`seam::close_through_pole`).
- Projection onto a spline with hundreds of spans (a helical flank) restarted
from a 16 x 16 grid and settled on another turn; it now starts from two
points per span, capped.
- Sag refinement no longer inserts a point exactly on a boundary side, which
split the side and failed the face.

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

* tess: a refused face's edges are divided finer and it is planned again (#2814)