Remove C++/Cython tribridge; add Python fallbacks and spectral features; simplify CLI/bench APIs

[jesusvilela]

Motivation

• Remove the compiled C++/Cython topology/GF(2) bridge and simplify the codebase to rely on pure-Python fallbacks and cheaper heuristics where appropriate.
• Replace brittle native build surface with lightweight, optional experimental features (numpy-backed spectral probe) and simplify benchmark/CLI patterns and parsing paths.

Description

• Deleted the C++/Cython topology and GF(2) bridge (backend/cpp/*, backend/cython/tribridge.*) and removed its Python wrapper (backend/cython/tribridge.pyx and generated files).
• Replaced heavy structural/tribridge calls with pure-Python fallbacks: removed the tribridge import in frame_solver.py parity path and simplified the critic in backend/acaf.py to a cheap num_vars-based hardness proxy rather than calling tribridge.
• Added experimental spectral features in backend/cnf_profile.py: implemented _shannon_entropy_ratio and _hyperbolic_delta_entropy_ratio (numpy optional) and wire CNFProfile.hyp_delta_entropy_ratio.
• Simplified DIMACS parsing in backend/cnf_utils.py by dropping direct .xz/.lzma handling from the parser wrapper to a simpler parse_dimacs_file (reads plain text), and removed an unused lzma import.
• Simplified benchmark discovery and CLI options: discover_benchmarks() now accepts a single pattern string (default "*.cnf") and backend/benchmark_cli.py accepts a single --pattern string instead of multiple patterns.
• Cleaned docs and README: removed Canary-specific/compiled-bridge claims, reorganized descriptive diagrams and component list, and updated quick-start and framing text.
• Small housekeeping: adjusted .gitignore (removed some duplicated entries / build lines).

Testing

• Ran the repository test suite for the backend: python -m pytest backend/tests/ -q; all tests completed successfully.
• Ran a few smoke CLI checks referenced in README (python -m backend.cli examples/simple_sat.cnf --mode strict and python -m backend.cli examples/simple_unsat.cnf --mode strict) to validate runtime paths after removing the native bridge; these commands executed without errors.

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Codex Task