feat(Analysis/Convex/SimplicialComplex): add AbstractSimplicialComplex + constructions

[BoltonBailey]

This adds the concept of abstract simplicial complex (and refactors SimplicialComplex in terms of it.

It also adds constructions that makes it easy to define a simplicial complex for any index family of points which is downward closed and which is affinely independent. I also include a construction of (abstract and geometric) simplicial complexes associated with a SimpleGraph, where vertices become 0-faces and edges become 1-faces, which could be useful later in defining the topological notion of a graph embedding.

Co-authored-by: Claude Opus 4.5

---

Notes: Perhaps it would be better to use stdSimplex as the underlying space for the graph?

[github-actions]

PR summary 1a6e4fde6a

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.Analysis.Convex.SimplicialComplex.AffineIndependentUnion (new file)	913

---

Declarations diff

+ AbstractSimplicialComplex
+ LinearIndependent.affineIndependent
+ instance : Min (AbstractSimplicialComplex ι)
+ instance : SemilatticeInf (AbstractSimplicialComplex ι)
+ ofAffineIndependent
+ onFinsupp
++ ofSimpleGraph

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[BoltonBailey]

I would be open to comments about whether I should refactor more of this to other files, and if so, where those files should live.