spec-guards differential: ring-096 REFUSED -- specs/numeric/formats.t27 has no function bodies #6893

What is left

Once the drift is fixed (#6887), ring-096-rust / specs/numeric/formats.t27 reads CONVERGED, so the spec-guards step "Run each CONVERGED pair on the same inputs" runs it. Measured on the t27c Railway lab at 6156948 (branch fix/spec-guards-ring-096):

python3 tools/ring_spec_differential.py rings/ring-096-rust specs/numeric/formats.t27
  stripped 2 inner doc-comment/attribute line(s) from rings/ring-096-rust/src/lib.rs so it can be included as a module

  covered 2 of 6 identical-signature functions
  REFUSED 4 -- these were NOT tested:
    f32_to_ternary: return type Trit
    format_bytes: parameter type Format
    quantize_value: parameter type Format
    ternary_to_f32: parameter type Trit
exit 2

This is an honest "could not test", not a pass and not a disagreement. There are three causes:

1. The spec has no bodies. All six functions in formats.t27 are fn ... { }, which gen-rust lowers to unimplemented!(). Even the two covered functions (gf16_to_f32, f32_to_gf16) can only disagree with the ring until the spec says what they compute. The SSOT bodies exist in specs/numeric/gf16.t27 (gf16_decode_to_f32, gf16_encode_f32).
2. The harness cannot build Format or Trit. Neither type has a producer, which is a spec function returning that type from grid-only parameters.
3. Harness detail. The blank line where the cases N agree N line should be is real. The producer call that builds the f32 argument (spec::gf16_to_f32) runs outside catch_unwind. It panics on unimplemented!(), so the whole harness process dies before printing. The exit code is still 2 because of the refusals, so nothing is hidden. But a "producer panicked" case should be reported as a difference, not as silence.

Done when

• formats.t27 either gets bodies or delegates to the gf16 SSOT functions, so gen-rust emits real code.
• ring_spec_differential.py rings/ring-096-rust specs/numeric/formats.t27 either exits 0 with all six functions covered, or names its disagreements.
• The harness reports a producer panic as a disagreement and no longer dies silently. This is a separate change to tools/ring_spec_differential.py, which is Python; under the only-t27 rule it needs the owner's label or a t27 port.

Until then, the spec-guards job is red at this step on any PR that reaches it. On master today, tools/published_figures.py --check fails earlier in the same job: 8 drifted figures, independent of this pair.

Refs #6887 #6488