Fix #255: Add SteinerTreeInGraphs model

[zazabap]

Summary

Add the SteinerTreeInGraphs problem model — a classical NP-complete network design problem about finding minimum-weight trees connecting terminal vertices in a graph.

Fixes #255

[codecov]

Codecov Report

❌ Patch coverage is 98.46743% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.62%. Comparing base (d309549) to head (3fb249c).

Files with missing lines	Patch %	Lines
src/models/graph/steiner_tree_in_graphs.rs	96.77%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #623      +/-   ##
==========================================
+ Coverage   96.61%   96.62%   +0.01%     
==========================================
  Files         218      220       +2     
  Lines       29352    29613     +261     
==========================================
+ Hits        28357    28613     +256     
- Misses        995     1000       +5     

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[zazabap]

Implementation Summary

Changes

• src/models/graph/steiner_tree_in_graphs.rs — New model implementation with SteinerTreeInGraphs<G, W> struct, Problem/OptimizationProblem trait impls, is_steiner_tree() connectivity checker, and declare_variants! with Dreyfus-Wagner complexity
• src/unit_tests/models/graph/steiner_tree_in_graphs.rs — 14 unit tests covering creation, evaluation (valid/invalid configs), direction, brute-force solver, serialization, size getters, single terminal edge case, all-terminals-are-spanning-tree degeneration, and the issue's 8-vertex example
• src/models/graph/mod.rs, src/models/mod.rs, src/lib.rs — Module registration and prelude export
• problemreductions-cli/src/dispatch.rs — load_problem and serialize_any_problem arms
• problemreductions-cli/src/problem_name.rs — Lowercase alias mapping
• problemreductions-cli/src/commands/create.rs — Create handler with --terminals flag, random generation, and parse_terminals() helper
• problemreductions-cli/src/cli.rs — --terminals CLI flag and help table entry
• docs/paper/reductions.typ — Problem definition with CeTZ figure showing optimal Steiner tree
• docs/paper/references.bib — Bibliography entries for Dreyfus-Wagner (1971), Bjorklund et al. (2007), Nederlof (2009)

Deviations from Plan

• Used SteinerTreeInGraphs as the name (matching the issue title and Garey & Johnson's canonical name) rather than MinimumSteinerTree suggested in the review comment
• Variables are per-edge (binary edge selection) as specified in the issue, matching the TravelingSalesman pattern

Open Questions

• None