Unify poison signal semantics across refs

[AnathemaOfficial]

Summary

This PR reseals the POISON_SIGNAL rule across the reference surfaces.

It aligns:

• the Rust reference sketch
• the Python reference sketch
• the reference documentation

What changed

• reject quantified_flow < 0 in the Rust reference sketch
• add a Rust test covering poisoned quantified_flow
• align the wording in the reference docs so POISON_SIGNAL means the same thing everywhere

Why

The audit found a drift between:

• docs/SIGNAL_PROVIDER_SPEC.md
• ref/README.md
• ref/syf_gate_ref.rs
• ref/syf_gate_ref.py

The docs and Python sketch treated negative quantified_flow as poisoned input, while the Rust sketch did not.

Result

The reference surfaces now agree on the fail-closed rule for poisoned signals.