docs(ra1n): add symbolic transfer bridge frontier

[inaciovasquez2020]

Summary

• adds RA1n symbolic transfer bridge status document
• records the finite-dimensional Gram-transversality bridge target
• adds verifier and pytest guard for conditional boundary language

Verified

• python3 scripts/verify_ra1n_symbolic_transfer_bridge.py
• python3 -m pytest -q tests/test_ra1n_symbolic_transfer_bridge.py
• python3 -m pytest -q

Boundary

• does not assert unconditional RA1n closure
• does not assert unrestricted conservation transfer
• records the weakest missing object: machine-checkable representation of the obstruction pairing by the same coordinate vector used in the finite Gram certificate