Remove stale structural-transformation internals from strict docs

[ChrisRackauckas-Claude]

Summary

The exact documentation build on current master reports 31 errors from
docs/src/internals/structural_transformation.md:

• 28 undefined bindings;
• no docstrings for find_solvables! and TearingState; and
• one dangling pantelides! cross-reference from the rendered
  dae_index_lowering docstring.

The page documents internal implementation names that were removed from or moved out of
ModelingToolkit. Re-exporting or qualifying those dependency internals would expand the
public dependency surface; the focused fix is to remove the stale entries and replace the
dangling pantelides! link with plain prose.

Reproduction

Tested at 92c27bb9fdcaa024ff190feb3d665452e33f9bd8 with Julia 1.12.6 on Linux x86_64:

/home/crackauc/.juliaup/bin/julia +release --project=docs --startup-file=no \
  -e 'using Pkg; Pkg.instantiate()'
/home/crackauc/.juliaup/bin/julia +release --project=docs --startup-file=no docs/make.jl

The second command exited 1. A local log aggregate found:

structural undefined=28
structural no_docs=2
structural crossrefs=1
all undefined=58
ERROR: LoadError: `makedocs` encountered errors [:docs_block, :missing_docs, :cross_references, :linkcheck]

The 28 undefined names are:

InducedCondensationGraph
MatchedCondensationGraph
Matching
SystemStructure
TransformationState
Unassigned
algeqs
algvars_range
check_consistency
computed_highest_diff_variables
dervars_range
diffvars_range
distribute_shift
get_fullvars
is_only_discrete
isalgvar
isdervar
isdiffeq
isdiffvar
linear_subsys_adjmat!
lower_shift_varname
pantelides!
shift2term
simplify_shifts
system_subset
tearing_reassemble
torn_system_jacobian_sparsity
unassigned

History

There are two relevant boundaries:

1. A representative source-content bisect for the removed isalgvar export used:

   git bisect start upstream/master ad3cd801ba70394adc6e950cc87a3318faf5ac03
   git bisect run bash -c 'git grep -Eq "export .*isalgvar" -- src && exit 0; exit 1'

   The first bad commit is fce566c46279c8945ed385f3ab7272cc17247b2c. It moved structural
   state-selection implementation into dependency packages but left this documentation
   page's blob unchanged from its parent.

2. The current hard-failure boundary is
   92c27bb9fdcaa024ff190feb3d665452e33f9bd8. Its parent kept
   [:docs_block, :missing_docs, :cross_references] in Documenter's warnonly; this commit
   removed that setting and exposed the pre-existing stale page as a hard failure.

Focused fix proof

I tested an uncommitted, deletion-only focused patch:

• remove the 28 undefined entries and the two non-owned/no-docstring entries from the page;
• remove the headings made empty by those deletions; and
• change [`pantelides!`](@ref) in dae_index_lowering to plain "internal Pantelides pass"
  prose.

The same full docs command then produced:

structural undefined=0
structural no_docs=0
structural crossrefs=0
all undefined=30

The full command still exited 1 on unrelated current-master docs errors in
bipartite_graph.md, API/model_building.md, API/problems.md, API/variables.md, and
other independent cross-references. I therefore did not push a first PR that is known to
fail the repository's required full-docs gate.

Additional local checks on the focused diff:

julia +release --startup-file=no -m Runic --check .     exit 0
typos docs/src/internals/structural_transformation.md \
  src/structural_transformation/pantelides.jl           exit 0
git diff --check                                         exit 0

Current-registry GROUP=QA completed with the independently tracked master result:
20 passed, 3 failed, 2 errored. The failures are JET, two ExplicitImports parser errors,
the undocumented StructuralTransformations module, and the unapproved
generate_trajectory re-export; they are unchanged by this docs-only fix and are tracked
separately.

Links

• Current-master failing docs job: https://github.com/SciML/ModelingToolkit.jl/actions/runs/31478304896/job/93752799681
• Immediately preceding green docs run: https://github.com/SciML/ModelingToolkit.jl/actions/runs/31474729578
• Underlying source-move commit: fce566c
• Underlying source-move PR: refactor: update to SymbolicUtils@4 and Symbolics@7 #3953
• Strict-docs boundary commit: 92c27bb
• Strict QA/docs PR: Adopt SciMLTesting 2.4 strict QA #4832
• Existing QA tracker: QA: track pre-existing Aqua/JET/ExplicitImports findings (root + ModelingToolkitBase) #4670

[ChrisRackauckas-Claude]

For maintainers, this is the exact focused diff used for the passing page-level
discriminator. It is not committed or pushed because current master still fails the full
docs command on independent pages.

diff --git a/docs/src/internals/structural_transformation.md b/docs/src/internals/structural_transformation.md
--- a/docs/src/internals/structural_transformation.md
+++ b/docs/src/internals/structural_transformation.md
@@
 tearing
-tearing_reassemble
 tearing_substitution
-torn_system_jacobian_sparsity
-find_solvables!
-linear_subsys_adjmat!
@@
 dae_index_lowering
-pantelides!
 pantelides_reassemble
 dummy_derivative

-## Consistency Checking

-@docs -check_consistency -
@@
lowest_order_variable_mask
highest_order_variable_mask
-computed_highest_diff_variables
-```

-## Shift Operations

-These functions handle shift operations in discrete-time systems.

-```@docs
-shift2term
-lower_shift_varname
-simplify_shifts
-distribute_shift

@@
-SystemStructure
-TearingState
-TransformationState
mtkcompile!
-isdiffvar
-isdervar
-isalgvar
-isdiffeq
-algeqs
-is_only_discrete
-dervars_range
-diffvars_range
-algvars_range
-get_fullvars
-system_subset
-```
-
-## Graph Types
-
-```@docs
-Matching
-InducedCondensationGraph
-MatchedCondensationGraph
-Unassigned
-unassigned

diff --git a/src/structural_transformation/pantelides.jl b/src/structural_transformation/pantelides.jl
--- a/src/structural_transformation/pantelides.jl
+++ b/src/structural_transformation/pantelides.jl
@@
Perform the Pantelides algorithm to transform a higher index DAE to an index 1
-DAE. kwargs are forwarded to pantelides!. End users are encouraged to call mtkcompile
-instead, which calls this function internally.
+DAE. kwargs configure the internal Pantelides pass. End users are encouraged to call
+mtkcompile instead, which calls this function internally.

Exact same-command aggregate after this diff:

```text
structural undefined=0
structural no_docs=0
structural crossrefs=0
all undefined=30

[ChrisRackauckas-Claude]

Validation started from then-current master 92c27bb9fdcaa024ff190feb3d665452e33f9bd8. A final fetch found master had advanced to 3061ef5b1d422d972cace7af58913c55d4a7f3ed while the long builds ran. That commit touches only docs/src/API/System.md; docs/src/internals/structural_transformation.md, src/structural_transformation/pantelides.jl, and docs/make.jl are byte-for-byte unchanged across the boundary, so the page-local reproduction and focused discriminator remain applicable. New master commit: 3061ef5