Add GP-node-discrete contour path (Phase 3.0e)

[nmorabowen]

Summary

Closes the directive's five-path dispatch (per docs/viewer/02-porting-from-apegmsh.md §4). Each element keeps its own corner-value copies — the cross-element averaging step is skipped, so visualisations preserve field discontinuities at element boundaries (material interfaces, plastic-hinge regions, local-stress jumps).

What lands

• make_gp_discrete_scalars in viewer/math/gauss_extrapolation.py: parallel to make_gp_nodal_scalars but skips extrapolate_to_nodes_averaged. Returns Callable[[step], dict[int, dict[int, float]]] — {element_id: {node_id: value}}. Same multi-step batch + 0-D defensive contracts as the smooth helper.
• ContourLayer.topology="nodal_discrete": routes through the same backend.add_polygons(point_values=...) call as "nodal" — the difference lives entirely in the per-vertex lookup. Duplicated vertex copies of a shared node hold the element-local value instead of the averaged value.
• Missing-entry error is topology-aware: "missing element id N" (outer key) vs "missing entry for (element id E, node id N)" (inner key).
• Topology validation message extended to list all three modes.

The five paths, all done

Path	Topology	Helper
nodal	"nodal"	(caller-supplied)
cell	"cell"	(caller-supplied)
GP-cell-averaged	"cell"	(caller pre-aggregates)
GP-node-extrap-smooth	"nodal"	make_gp_nodal_scalars
GP-node-discrete	"nodal_discrete"	make_gp_discrete_scalars

Test plan

• 7 new gauss unit tests: constant-field nested dict, two-quad per-element preserved (not averaged), step-varying callable, empty lookup, 0-D input rejected, multi-step batch indexing, out-of-range raises.
• 6 new PyVista-gated integration tests: discrete-topology acceptance, mpl capability error, two-quad value preservation at shared edges, in-place update_to_step on the SAME actor, missing-element and missing-(element,node) error messages, end-to-end via make_gp_discrete_scalars.
• 2 new mpl-only tests: topology validation message lists all modes; discrete mode on mpl raises BackendCapabilityError with the "per-vertex" hint.
• system Python (no pyvista): full suite 1629 passed, 54 skipped (+10 over the 1619 baseline; 6 new pyvista-gated tests skip correctly).
• opensees_venv (pyvista 0.47.1 + VTK 9.6.1): combined contour + gauss suite 81 passed, including all 6 new PyVista integration tests.

What's next

Phase 3 contour story is complete. Followup: bump pyproject.toml + tag a release on main per CLAUDE.md's versioning policy.

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