test: Add cross runtime protocol conformance matrix (Phase 2)

[Titas-Ghosh]

Hi @pradeeban,
Following up on the discussion in #440 and Phase 1 conformance work, here is the PR for Phase 2 (mapping/audit).

As promised in our initial plan, this PR is strictly report only and non breaking. It introduces a language agnostic matrix to track how different runtimes handle the protocol cases we defined in Phase 1, without changing a single line of runtime logic anywhere.

What Changed

• The Matrix (cross_runtime_matrix.phase2.json): Added phase 2 metadata (phase, mode, runtimes, classifications, statuses).
  • Mapped each Phase 1 case across python, cpp, matlab, octave, and verilog.
  • Crucially, all non Python runtimes are explicitly marked as not_audited to ensure this phase remains purely observational.
• The Runner (tests/test_protocol_conformance_phase2.py):
  • Validates the matrix metadata and enums.
  • Ensures a 1:1 mapping with Phase 1 case IDs.
  • Enforces row shape consistency and verifies the python=observed_pass baseline.
• Documentation (PROTOCOL_FIXTURES.md): Updated to clearly document the Phase 2 scope and constraints.

Safety

• Zero production/runtime files were modified. There are absolutely no behavior changes for any existing language bindings.
• We are not forcing cross-language behavior; this just adds the structured audit foundation so we can review the landscape before considering any future enforcement.

Validation

• Targeted conformance tests:
  python -m pytest -v tests/test_protocol_conformance.py tests/test_protocol_conformance_phase2.py → 10 passed
• Broader non-regression run:
  python -m pytest -v tests --ignore=tests/test_openjupyter_security.py → 99 passed

Evidence

1) Phase-2 matrix metadata (report only + enums)

2) Example mapped case (Python observed, others pending)

3) Targeted conformance tests passing

4) Broader test run passing

Note to Maintainers

I kept this intentionally minimal and report only so we can review the matrix structure together before we start classifying how the other languages currently behave. let me know if this structure looks good to you.