fix(cloudxr): start runtime with spawn context

[WilliamK112]

Summary

• start the native CloudXR runtime process from an explicit spawn multiprocessing context instead of the platform-default fork context
• preserve the existing multiprocessing.Process handle contract used by readiness polling and terminate_or_kill_runtime
• add a focused unit test that stubs native CloudXR imports and verifies the spawn context
• document the process-model decision in the architecture changelog

Rationale

Issue #40 calls out that the current multiprocessing.Process(target=runtime_run) path can fork from inside an already-running asyncio event loop. Using spawn starts the runtime in a fresh interpreter, avoiding inherited asyncio lock and signal-handler state while keeping the current readiness and cleanup APIs intact.

Fixes #40

Validation

Refreshed onto current NVIDIA/xr-ai@6de6ca73cec931c5833fe7c418fb30e1df5b4172; GitHub reports behind_by: 0, ahead_by: 1.

• pytest -q tests/test_cloudxr_runtime_process.py tests/test_cloudxr_service_layout.py — 4 passed
• Ruff 0.15.16 on both changed Python files — passed
• repository SPDX checker on source, test, and changelog — passed
• signed-off commit / DCO — present
• git diff --check upstream/main...HEAD — passed

The unit test imports the runtime from its current services/cloudxr-runtime location and replaces the platform-specific Isaac Teleop modules with fakes, so this process-launch contract is verified without requiring CloudXR hardware.

[WilliamK112]

Refreshed this PR onto current main (6de6ca73) after CloudXR moved under services/. The regression test now imports the current service path and stubs the newly required launcher API, and the fork-to-spawn rationale is recorded in docs/changelog.md. The PR remains one signed commit; GitHub reports behind_by: 0, ahead_by: 1, and mergeable. Validation on c53795e1: 4 related tests passed, Ruff/SPDX/DCO/diff checks clean.

[WilliamK112]

Refreshed onto current main (b641bf59) and resolved the changelog-only conflict while preserving all newer upstream decisions. Validation on ad812f6: 4 focused tests passed; Ruff, SPDX, DCO, and git diff --check all pass.