Unify poison signal semantics across refs

[Copilot]

PR #3 carried the right changes but was blocked on Commits must have verified signatures. This re-applies the identical diff on a verified-signature commit.

The drift

The Rust gate checked only r_local < 0 and quantified_entropy < 0 for poison detection — silently passing a negative quantified_flow. Docs and the Python sketch already treated all three fields as poison triggers.

Changes

• ref/syf_gate_ref.rs

  • Add quantified_flow < 0 to TV-G-003 rejection guard:

    if input.signal().r_local() < 0
        || input.signal().quantified_flow() < 0
        || input.signal().quantified_entropy() < 0

  • Add tv_g_003_invalid_signal_flow test covering the gap
  • Update Signal and POISON_SIGNAL doc-comments to list all three poison fields
  • Drop unused use core::fmt

• docs/SIGNAL_PROVIDER_SPEC.md

  • Failure-mode diagram: Gate evaluates: signal.r_local < 0 → Gate evaluates: any negative poison field

• ref/README.md

  • Trim trailing whitespace on the Canonical Rule line
  • Rust constraints: Deterministic SHA-256 for finality_tag only → Neutral fixed finality tag in the P0 sketch
  • Python constraints: correct import list (dataclasses, typing, enum — no hashlib)

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