Add graph theory subsystem (src/graph/)

[msollami]

Summary

Adds a Mathematica-compatible graph theory subsystem (src/graph/) to
Mathilda, modelled on the Wolfram Language and mirroring the existing
src/linalg/ layout (one builtin per translation unit, registered via
graph_init()). Graphs are ordinary Expr trees —
Graph[List[verts], List[edges]] with each edge a DirectedEdge[u, v] or
UndirectedEdge[u, v] — so the generic tools (Part, Map, ReplaceAll,
pattern matching) work on them unchanged. Rule/-> and TwoWayRule/<->
are accepted as parse-time sugar and normalised on construction.

Builds cleanly under the existing toolchain (-std=c99 -Wall -Wextra,
USE_GRAPHICS/USE_LAPACK/USE_MPFR/USE_ECM) with no changes to any
existing subsystem's behaviour. PageRank and KatzCentrality are computed
exactly by assembling a rational matrix and calling the existing
LinearSolve.

Changes

• src/graph/ (new, 103 files): construction/predicates (Graph, GraphQ,
  DirectedGraphQ, TreeGraphQ, BipartiteQ, ConnectedGraphQ, …); queries
  (VertexList, EdgeList, VertexCount, VertexDegree, DegreeSequence,
  AdjacencyList, GraphDensity, …); matrix views (AdjacencyMatrix,
  IncidenceMatrix, KirchhoffMatrix, DistanceMatrix); structural ops
  (GraphComplement, GraphUnion, GraphProduct, LineGraph,
  TransitiveClosureGraph, vertex/edge add/delete/contract, …); generators
  (CompleteGraph, CycleGraph, CirculantGraph, GeneralizedPetersenGraph,
  PrismGraph, RandomGraph, …); algorithms (FindShortestPath,
  ConnectedComponents, FindSpanningTree, VertexConnectivity,
  FindEulerianCycle, FindHamiltonianCycle, FindClique,
  FindVertexCover, FindGraphMatching, FindVertexColoring,
  ChromaticNumber, ChromaticPolynomial, …); centrality (PageRank,
  KatzCentrality, BetweennessCentrality, ClosenessCentrality,
  clustering coefficients); and drawing (GraphPlot → Graphics,
  Graph3D/GraphPlot3D → Graphics3D, HighlightGraph).
• src/core.c: register graph_init() after graphics_init().
• src/sym_names.{c,h}: intern Graph/Graph3D/DirectedEdge/
  UndirectedEdge/TwoWayRule and graph-drawing option symbols.
• src/parse.c: add the <-> (TwoWayRule) operator at Rule precedence.
• src/print.c: print edges infix (u -> v / u <-> v) and a bare Graph
  as a terse Graph[<n vertices, m edges>] summary; InputForm/FullForm
  round-trip through the literal constructor.
• src/graphics/graphics_json.c: graph-diagram serialisation
  (graphics3d_to_plotly_json, diagram mode for Disk/Point/Text); a bare
  valid Graph/Graph3D result auto-renders like Graphics.
• makefile: compile src/graph/*.c, add -I./src/graph.
• tests/CMakeLists.txt: build the graph_tests target.
• Docs: new docs/spec/builtins/graphs.md; index row in Mathilda_spec.md;
  layout/init notes in SPEC.md; changelog entry under
  docs/spec/changelog/2026-06-29.md.

Testing

• make -j — clean build, ./Mathilda links with graphics/LAPACK/MPFR/ECM,
  zero warnings from src/graph/.
• cd tests && cmake -B build && cmake --build build --target graph_tests then
  ./build/graph_tests → all graph tests pass (1000+ assert_eval_eq
  assertions across ~90 test functions, exercising parse → eval → print,
  including <->/-> edge sugar).

JIRA Ticket

N/A