Add VNAlgorithm dispatch for very_nauty C library algorithms

[nenadilic84]

Summary

Adds a VNAlgorithm dispatch type that routes graph algorithms to the very_nauty C library implementation, following the pattern specified in JuliaGraphs/Graphs.jl#448.

New API

using VNGraphs
import Graphs

g = Graphs.cycle_graph(5)

# Dispatch to very_nauty via VNAlgorithm()
Graphs.is_connected(g, VNAlgorithm())        # true
Graphs.connected_components(g, VNAlgorithm()) # [[1, 2, 3, 4, 5]]

# very_nauty-specific algorithms (not in Graphs.jl)
chromatic_number(g, VNAlgorithm())       # 3  (exact, via exhaustive search)
edge_chromatic_number(g, VNAlgorithm())  # 3
clique_number(g, VNAlgorithm())          # 2

Design

• VNAlgorithm is an exported dispatch type
• All methods accept any AbstractGraph and auto-convert to VNGraph internally when needed
• is_connected and connected_components extend existing Graphs.jl functions
• chromatic_number, edge_chromatic_number, and clique_number are new functions exported from VNGraphs (these algorithms don't exist in Graphs.jl yet)

Algorithms dispatched

Function	C library call	Notes
Graphs.is_connected(g, ::VNAlgorithm)	graph_connected	Returns Bool
Graphs.connected_components(g, ::VNAlgorithm)	graph_nclusters + cluster labels	Returns Vector{Vector{Int}}
chromatic_number(g, ::VNAlgorithm; timeout=0)	graph_chromatic_number	Exact, optional timeout
edge_chromatic_number(g, ::VNAlgorithm; timeout=0)	graph_edge_chromatic_number	Exact, optional timeout
clique_number(g, ::VNAlgorithm)	graph_clique_number	Exact

Bounty checklist progress (Graphs.jl#448)

• Dispatch from operations to very_nauty implementation via VNAlgorithm
• Declare missing algorithms in Graphs.jl (chromatic_number, clique_number — separate PR to Graphs.jl)

All 307 tests pass locally on Julia 1.11.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.52%. Comparing base (b1faf64) to head (8e2947f).

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #19       +/-   ##
===========================================
+ Coverage   45.90%   86.52%   +40.62%     
===========================================
  Files           1        1               
  Lines          61      141       +80     
===========================================
+ Hits           28      122       +94     
+ Misses         33       19       -14     

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[nenadilic84]

All CI test jobs pass across all platforms (ubuntu, windows, macOS x64, macOS aarch64).

The only failing check is Documentation — same pre-existing issue as PR #18 (no docs/make.jl in the repo).