pet-2: extract guarded redirect backend

[gcomneno]

Closes #134

Summary:

• Extracts guarded structural redirect execution logic into stable package code.
• Updates the research probe to delegate to the package backend.
• Updates structural-factorization --guarded-redirect to use package code directly instead of calling the research probe.
• Adds direct backend regression tests for the known flat-k redirect case and completed lateral-door control.

Validation:

• uv run pytest -q tests/test_guarded_redirect.py tests/test_tools_pet_guarded_redirect_execution_probe.py tests/test_cli_structural_factorization.py -rs
  • 17 passed
• python -m py_compile src/pet/cli.py src/pet/guarded_redirect.py tools/research/pet_guarded_redirect_execution_probe.py tests/test_guarded_redirect.py
• git diff --check

Boundary:

• No default routing change.
• No anchor-selection change.
• No factorization-performance claim.
• No lookup tables or per-number trap catalogues.