Add variant-level aliases; introduce 2SAT and 3SAT

[GiggleLiu]

Summary

Aliases in this codebase used to resolve only to a canonical problem name, never to a specific variant. That forced 3SAT to be excluded as an alias (see the old comment at problem_name.rs), because a plain alias 3SAT → KSatisfiability would silently give users the default variant KN, not K3.

This PR makes aliases a first-class per-variant concept so shorthand like 3SAT lines up with the actual reduction-graph node (KSatisfiability<K3>), not just the problem type. Problem-level aliases (MIS, SAT, MVC, MAX2SAT, ...) are unchanged.

What changes

• VariantEntry gains aliases: &'static [&'static str].
• declare_variants! accepts optional trailing aliases ["X", ...]:

  KSatisfiability<K2> => "num_variables + num_clauses" aliases ["2SAT"],
  KSatisfiability<K3> => "1.307^num_variables" aliases ["3SAT"],

• registry::find_variant_by_alias(input) -> Option<(&VariantEntry, BTreeMap<_, _>)>
  — returns the matched entry and its variant map.
• CLI parse_problem_spec and resolve_alias try variant-level aliases first,
  injecting the alias's variant tokens into the parsed spec. Problem-level alias
  resolution is untouched.
• pred list shows variant-level aliases on their own rows (e.g., KSatisfiability/K2 shows 2SAT, KSatisfiability/K3 shows 3SAT).

Scope

Only the three aliases the user asked for: 2SAT, 3SAT, MAX2SAT. MAX2SAT was already a problem-level alias on the standalone Maximum2Satisfiability type and is left untouched. No other aliases changed.

End-to-end verification

$ pred show 3SAT   → KSatisfiability/K3 (Best Known Complexity: O(1.307^num_variables))
$ pred show 2SAT   → KSatisfiability/K2 (Best Known Complexity: O(num_clauses + num_variables))
$ pred show MAX2SAT → Maximum2Satisfiability (unchanged)
$ pred path 3SAT MaximumIndependentSet → 3-step path starting from KSatisfiability/K3

Test plan

• cargo fmt --check
• cargo clippy --all-targets --features ilp-highs -- -D warnings
• make test (5092 unit tests + all integration/doc tests pass)
• End-to-end: pred show {3SAT,2SAT,MAX2SAT} resolve correctly
• End-to-end: pred path 3SAT MaximumIndependentSet finds a path from K3
• End-to-end: pred list shows 2SAT/3SAT on the correct variant rows

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 98.13665% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.92%. Comparing base (e38b1b5) to head (de23124).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/unit_tests/registry/variant.rs	96.34%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1054      +/-   ##
==========================================
- Coverage   97.94%   97.92%   -0.03%     
==========================================
  Files         979      964      -15     
  Lines      100890    99869    -1021     
==========================================
- Hits        98816    97793    -1023     
- Misses       2074     2076       +2     

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