Refactor unweighted gadget search around exact rewrites

[isPANN]

Summary

• replace the fixed lattice-only beam with a dynamic logical-graph search
• require alternating planar four-pin topology before lattice work
• guide repairs with independent-set witnesses, then use tensor-preserving vertex splits and even subdivisions
• embed only final rewrite candidates on KSG or triangular lattices and accept with the unchanged verifier plus G1-G4
• remove graph6 from the search state and keep readable edge-list histories

Verification

• julia --project=. -e "using Pkg; Pkg.test()"
• fixed verifier section is byte-identical to main
• 13-vertex logical CROSS passes the verifier
• 37-site triangular CROSS passes the verifier, G1-G4, and induced embedding search

Current limitation

The new pipeline has not yet rediscovered the triangular CROSS end-to-end from a random start within the tested 50k/100k logical budgets. Four fresh 100k runs and four fresh 50k runs reached one reduced-alpha boundary-state mismatch. This PR is intentionally draft while the logical plateau and joint subdivision search are improved.

[codecov]

Codecov Report

❌ Patch coverage is 88.73239% with 64 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/core/unweighted_search.jl	88.73%	64 Missing ⚠️

Files with missing lines	Coverage Δ
src/core/unweighted_search.jl	88.09% <88.73%> (-8.83%)	⬇️

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