feat: prove portable execution across Python versions

[byte271]

What changed

This draft introduces an isolated cross-Python execution-image experiment without weakening the shipping exact-version restore path.

• adds continuum-execution-abi-1.0 as an experimental compatibility contract
• writes checksummed portable images with an explicit allowlist for CPython 3.12.13 and 3.13.14
• keeps creator runtime/Python identity as provenance while allowing restore by a runtime implementing the same execution ABI
• adds refusal tests for unknown Python versions and keeps the ordinary exact-version reader intentionally incompatible with the experimental capability
• adds a two-job proof workflow: Linux x86_64 / CPython 3.12.13 source, then native macOS arm64 / CPython 3.13.14 target
• retains image hashes, frame/graph evidence, exactly-once action evidence, and uninterrupted-control comparison

Why

The current image contract requires exact CPython 3.12.13 and the exact Continuum runtime version. That means even a compatible host runtime cannot restore an image after a Python or Continuum version change. The experiment tests whether Continuum's explicit semantic frames and IR can form a stable execution ABI instead of inheriting CPython bytecode-version coupling.

Proof gate

This PR must not be described as working until Cross-Python portable execution proof passes end to end. The required result is one byte-identical image moving from Linux x86_64 / Python 3.12.13 to macOS arm64 / Python 3.13.14, restoring four active logical frames, preserving shared references and a cycle, repeating zero completed actions, and matching an uninterrupted control.

Scope

This is deliberately a feasibility path beside the existing image reader. It does not change the public CLI, package version, shipping format claim, or existing exact-version compatibility rules. Promotion into a release requires the documented gate and a later integration pass.

[coderabbitai]

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[byte271]

First cross-Python proof passed

Actions run 30655561871 completed successfully.

• source: native Linux x86_64, CPython 3.12.13
• target: native macOS arm64, CPython 3.13.14
• source job completed before the dependent target job
• transferred image remained byte-identical: 915bd5f2602d6d3ae163fa698d0253ef67b1b643e4da315b5ea45fb58a2de814
• four live logical frames restored: __module__ -> outer -> middle -> leaf
• shared reference and reference cycle preserved
• completed output actions repeated: 0
• source + target output matched the uninterrupted control
• combined/control output SHA-256: 57e199c47be9aa8fb3618745192790e7e303c2962947a8ee49a861d83d0beef9
• all five portable-image acceptance/refusal tests passed under both Python versions

This proves the selected controlled workload and experimental reader. It is not yet a general cross-version support claim.

[byte271]

Closing this draft as superseded.

The execution-ABI work this PR prototypes shipped through #2 and #3 instead. main is now at 39cc64f carrying Execution ABI 1.0, container FORMAT_VERSION 0.2 with a 0.1 legacy path, IR 0.4, and two-sided admissibility gating on abi.VERIFIED_PYTHON_VERSIONS / abi.VERIFIED_PLATFORMS — a superset of what is proposed here.

Evaluated against current main rather than the main it was branched from, this branch is no longer a candidate to merge:

• It does not merge cleanly as a change; it reintroduces a parallel implementation of ~1,090 lines alongside the one already shipped.
• Applied on top of current main it does not pass: 318 tests, 4 failures and 5 errors. All five errors are KeyError: 'target_compatibility' from portable_image.py:136, which still assumes the format-0.1 manifest shape that feat: productionize the cross-Python Execution ABI (0.4.0a1) #2 replaced.
• Its own targeted repeats (windows-2025) check on this PR is red.

None of that is a defect in the experiment — it answered its question, and the answer is in main. The branch cross-python-runtime-v0.4 (fc145a5) is being left in place rather than deleted, so the history stays reachable.