[udaykusupati]

Follows #1155 (envelope reference).

What this does

Tags become the one way components hand each other what they hold. Each component writes its tracked faces, tracked curves and feature curves as tags, on both routes (memory and file). A reader turns the tags back into tracked simplices with their input sets. Before this PR, several of these were lost between components.

Component	Writes
tetwild	Oriented face tags, one per input (input_names, else tag_<i>), as triangle blocks of the memory mesh and as physical groups of _final.msh. The winding numbers wn_<i> stay fields. No volume tags.
tetwild, simwild 3D	Feature curves (the tet edges that tile each input_edges file) as an edge block, feature_<i> per file. The order2 kind is not written; a reader derives it again.
triwild	Each input's curves as edge tags, named as its triangle tag and oriented as the input.

Reader (read_mesh_data(), read_image_msh())	Reads
2D edge block	Tracked curves, with their input sets and orientation. EnvelopeSurface rows (or a block without tags) are the envelope, as before.
3D edge block	Feature curves, held as input_edges are.
File route	Keeps corr_input_vid (and vid), as the memory route always did. A chain of offsets keeps its correspondence through files.

Envelope passing

A pipeline (tetwild → simwild remeshing → …) used to keep its guarantee only against the previous stage's output: each stage built its envelopes around the mesh it received, so the bound against the original input grew by one eps per stage. Now the first component's reference (the input, or the simplified input, per envelope_reference) is passed on with its labels, and a receiver holds to it. The bound stays one eps of the input.

	Memory route	File route
Producers (tetwild, triwild, simwild)	outputs["envelope"], now labelled	<output>_envelope.msh (new)
envelope key of a receiver	a mesh name	a path (new; it was ignored on the file route)

• The envelope: a mesh of the reference, labelled as the tags are: triangles (2D: segments) with one column per input; the feature curves' segments as an edge block; input points as an untagged point block. No eps.
• simwild remeshing with an envelope: holds each face to its labels' reference envelopes and each feature edge to tubes around the reference's curves, with its own eps. At the start it checks, exactly, that every face and feature edge it received lies inside those envelopes; otherwise it throws with the label, the count outside and the eps. A label the envelope lacks throws.
• Pass-through: a receiver hands on the envelope it was given, unchanged. topological_offset and manifold_extraction read and hand it on; neither holds to it yet.
• Not checked: order-2 tubes and curve-end anchors. The engine holds an edge the reference lacks around itself, which the mesh cannot tell apart from a wider sender.

Behaviour changes

• tetwild with several inputs and per_input_envelopes off: each tracked face's input set now comes from the arrangement, because the face tags need it. Before, every face was input 0.
• 2D edge blocks with labels other than EnvelopeSurface: these are now tracked curves. Before, any such label made the whole block the envelope. No data model relies on the old behaviour: the only 2D edge group in data2 is EnvelopeSurface (models/topo_annots_groups.msh).
• topological_offset and manifold_extraction: given an input with feature curves, they now refuse it instead of silently dropping the curves. Neither holds feature curves yet.
• tetwild's memory mesh can no longer be written to .vtu: the VTU writer refuses labelled fields, as it already did for simwild's and triwild's meshes. The quickstart example now writes .msh. Making the VTU writer skip tag blocks is a follow-up.

Tests

• tetwild writes each input's face tags and its feature curves, on both routes.
• triwild writes each input's curve tags, on both routes.
• A tagged mesh's edge block reads as tagged curves (2D) and as feature curves (3D).
• simwild keeps a tagged mesh's curves, on both routes.
• Tagged mesh files carry an earlier offset's correspondence.
• Two tests assumed tetwild writes no triangles of its own. They now match faces by their corners.

Verification

Run on this branch, whose base has the same tree as main at ab4fe46, with the envelope passing:

• All 9 C++ suites pass: wmtk_tests, wmtk_component_io_tests, tetwild, simwild, triwild, tracking, topological_offset, shortest_edge_collapse, manifold_extraction.
• The 15 docs examples pass.
• python/tests: 61 passed, 4 skipped (the PolyFEM tests, which need POLYFEM_BIN), including the envelope chain on both routes. One test asserted that triwild's file holds no edge block, which stood for "the file holds no envelope"; it now checks for no EnvelopeSurface rows.
• tests/python: 110 passed, including the 62 docs-example subtests. One collection error appears on both this branch and main: pytest collects the helper tested_elsewhere() as a test.

🤖 Generated with Claude Code