Add method to find connected components of SolidModels entities

[gpeairs]

Close #151 under the assumption that connected subgroups only need to be identified, not used in further postrender operations. Implements SolidModel.connected_components which takes physical group names or entity tags and a dimension and outputs a vector of the connected components in those groups or tags.

To create physical groups, you would run something like this after rendering:

metal_islands = SolidModels.connected_components(sm, "metal")
for i in eachindex(metal_islands)
    sm["metal_island_$i"] = metal_islands[i]
end

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[ybrightye]

Bug: return type mismatch in early exits of connected_components(dim, tags)

The two early returns in connected_components(dim::Int, tags::Vector{Int32}) return Vector{Vector{Int32}}, but the main code path returns Vector{Vector{Tuple{Int32, Int32}}} (dimtags):

isempty(tags) && return Vector{Int32}[]   # Vector{Vector{Int32}}
n == 1 && return [tags]                    # Vector{Vector{Int32}}

Should be:

isempty(tags) && return Vector{Tuple{Int32,Int32}}[]
n == 1 && return [[(Int32(dim), tags[1])]]

The "empty input" and "single entity" tests currently pass because they assert the buggy return type (result[1] == Int32[1]). They'd need updating too.

[gpeairs]

Bug: return type mismatch in early exits of connected_components(dim, tags)

Thanks, fixed.

[ybrightye]

Validated: flux loop workflow works end-to-end

Ran the following script against the PR branch (cherry-picked onto gp/channels-dev for compat with the ExamplePDK):

test/demo_connected_components.jl (click to expand)

using DeviceLayout, DeviceLayout.PreferredUnits, DeviceLayout.SolidModels
import DeviceLayout.SolidModels: connected_components
using DeviceLayout.SchematicDrivenLayout
using DeviceLayout.SchematicDrivenLayout.ExamplePDK
import DeviceLayout.SchematicDrivenLayout.ExamplePDK: LayerVocabulary

g = SchematicGraph("single-transmon")
q = ExamplePDK.Transmons.ExampleRectangleTransmon()
rr = ExamplePDK.ReadoutResonators.ExampleClawedMeanderReadout()
qubit_node = add_node!(g, q)
rres_node = fuse!(g, qubit_node, rr)
p_readout = Path(Point(0μm, 0μm); α0=π/2, name="p_ro", metadata=LayerVocabulary.METAL_NEGATIVE)
straight!(p_readout, 2mm, Paths.CPW(10μm, 6μm))
straight!(p_readout, 2mm, Paths.CPW(10μm, 6μm))
csport = CoordinateSystem(uniquename("port"), nm)
render!(csport, only_simulated(centered(Rectangle(10μm, 10μm))), LayerVocabulary.PORT)
attach!(p_readout, sref(csport), 10μm, i=1)
attach!(p_readout, sref(csport), 2mm - 10μm, i=2)
p_readout_node = add_node!(g, p_readout)
attach!(g, p_readout_node, rres_node => :feedline, 0mm, location=1)
floorplan = plan(g)
chip = offset(bounds(floorplan), 2mm)[1]
render!(floorplan.coordinate_system, chip, LayerVocabulary.SIMULATED_AREA)
render!(floorplan.coordinate_system, chip, LayerVocabulary.WRITEABLE_AREA)
render!(floorplan.coordinate_system, chip, LayerVocabulary.CHIP_AREA)
check!(floorplan)

sm = SolidModel("test", overwrite=true)
render!(sm, floorplan, ExamplePDK.singlechip_solidmodel_target(["port_1","port_2","lumped_element"]); strict=:no)

# --- Detect connected components ---
metal_comps = connected_components(sm, "metal")
active_comps = connected_components(sm, ["metal", "lumped_element", "port_1", "port_2"])

# --- Assign as named groups (flux loop workflow from issue #151) ---
for i in eachindex(metal_comps)
    sm["metal_island_$i"] = metal_comps[i]
end

@assert length(metal_comps) == 3  # ground, island, CPW trace
@assert length(active_comps) == 1  # all connected through ports
for i in 1:3
    @assert haskey(SolidModels.dimgroupdict(sm, 2), "metal_island_$i")
end

Output:

metal: 7 entities → 3 connected components
metal + lumped_element + ports: 1 connected component(s)

metal_island_1: 1 entities, grouptag=41
metal_island_2: 2 entities, grouptag=42
metal_island_3: 4 entities, grouptag=43

✓ All assertions passed

This validates the real use case from #151: automatically detecting disconnected regions of a physical group and assigning them as named subgroups for Palace simulation config. Previously this required hardcoding the number of subgroups (e.g., for i in 1:3 in our magnetostatic setup scripts).

Suggestion: consider adding this as a test alongside the existing metal_conn_comps assertions in test_examples.jl — specifically the sm["metal_island_$i"] = metal_comps[i] assignment + retrieval, since that's the workflow users actually need and it exercises the setindex! path with dimtags from connected_components.

Also noticed commit 1231b34 fixed the return type bug I flagged earlier — thanks!

[gpeairs]

Suggestion: consider adding this as a test alongside the existing metal_conn_comps assertions in test_examples.jl — specifically the sm["metal_island_$i"] = metal_comps[i] assignment + retrieval, since that's the workflow users actually need and it exercises the setindex! path with dimtags from connected_components.

Good call, added.