feat: interop test package + session handoff doc

Cross-spec interop validation between ietf-act and ietf-ect:
- new packages/interop/ sibling package (ietf-act-ect-interop)
- 32 tests pass: shared claims, algorithm matrix, DAG structure,
  divergence handling, anti-goals
- documents ES256 raw signature wire-compatibility
- documents airtight typ separation (act+jwt vs exec+jwt)

Hazards surfaced:
- ACTLedger.append() silently accepts ECT Payload via duck-typing
  (both have .jti) — documented in interop README as a production
  hazard requiring external isinstance checks

Session handoff:
- SESSION-2026-04-12.md — snapshot of decisions, artifacts, open
  actions, and next-session starting points

Also: session-end commit of hash-format fix propagation to
packages/ect/ (the fix was applied to the old refimpl location
but did not propagate through the parallel package-move agent).

workspace/packages/interop/pyproject.toml

@@ -0,0 +1,22 @@
+[build-system]
+requires = ["setuptools>=68.0"]
+build-backend = "setuptools.build_meta"
+
+[project]
+name = "ietf-act-ect-interop"
+version = "0.1.0"
+description = "Cross-spec interop tests between ietf-act and ietf-ect reference implementations"
+requires-python = ">=3.11"
+license = "BSD-3-Clause"
+dependencies = [
+    "ietf-act",
+    "ietf-ect",
+    "pytest>=8.0",
+]
+
+[tool.setuptools.packages.find]
+where = ["."]
+include = ["tests*"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]