ECS0System ProcessRunner portability source commit

Prepared a production-mode isolated ecs0lib commit that preserves macOS subprocess behavior while making ProcessRunner truthfully importable on iOS, tvOS, and visionOS with an explicit unsupported capability and error contract.

• Host: rdmsm4x
• Task: TASK-20260903-07
• Scope: isolated worktree /Users/richh/dev/_worktrees/ecs0lib-processrunner-portable-20260903, branch codex/ecs0lib-processrunner-portable-20260903, base 8805372710261de772577fbe9a44f6eff445a9e3.
• Commit: 00a0fada5fa1f88446fb334342fa619176ed47fb.
• Files changed: Sources/ECS0System/ProcessRunner.swift; Tests/ECS0LibTests/ProcessRunnerPortabilityTests.swift.
• Verification: focused tests 3/3; ThreadSanitizer 299 XCTest plus 11 Swift Testing with zero failures and no reported race; ECS0System generic macOS, iOS, tvOS, and visionOS builds all exit 0; full macOS aggregate exits 0.
• Independent blockers: mobile full aggregates on exact base 8805372 still fail in the separately owned CECSNetTopBridge because libproc.h is macOS-only; tvOS also fails the pre-existing LocalAuthentication import. The primary will cherry-pick this commit onto FEAT-20260902-17 and rerun the integrated aggregate.
• Backup and evidence: /Users/richh/dev/_handoff/rtty-appstore-finalization-20260902/xcode-repair/TASK-20260903-07-REPORT.md, SHA-256 9aa6ff5d90ba6b66996a906c80dc2eb2cf9bef49442fda2e3c9061e0ea4f339c.
• Undo: before integration, discard the isolated branch/worktree; after integration, revert the cherry-picked commit without rewriting shared history.
• Outstanding owner action: cherry-pick 00a0fad onto FEAT-17, rerun TSan and all four aggregate builds, then separately route tvOS LocalAuthentication if it remains.

No canonical ecs0lib worktree, RTTy source, installed app, global Xcode selection, signing identity, deployment, or public state was changed.