[astrobyte-dev]

The P2.5 checkpoint ran shared CI on Ubuntu but still used Windows defaults, lacked Linux descendant cleanup, and could not open the default Linux browser or report native process/socket health. This adds a bounded P2.6 foundation while retaining the persistent owner and Windows implementations.

Linux now uses workspace-specific XDG state, native executable discovery, /proc inventory, dedicated managed child sessions, creation identities and pidfd-based TERM/KILL cleanup with exit/socket checks. Browser opening uses xdg-open and leaves a healthy runtime running if opening fails. System reports platform/backend capabilities and unavailable inspection honestly; JACK mode avoids unsupported device enumeration and Windows buffer/rate defaults. Recording-aware lifecycle remains shared, including graceful Linux signals and ephemeral MCP shutdown.

Validation on Ubuntu 24.04.4 with Node 24.14.0/npm 11.9.0: production build and both typechecks pass; 174 tests pass, three existing native Windows tests skip (177 discovered, 20 new). Classic/Studio/P2/System browser journeys pass. Real Node process/descendant fixtures cover identity rejection, separate sessions, deadline/escalation, unrelated-process preservation, port release and Stop/Restart/Quit. The Linux runtime harness verifies cold/second launch, unchanged session, native inventory, browser-failure fallback, signal shutdown and external conflict protection. Actual xdg-open produced the expected Studio request. CI now runs shared checks on Ubuntu and Windows.

Exit level: B — Ubuntu Runtime Safe, using native Node fixtures/runtime. Level C native audio is NOT validated. The machine's pre-existing half-configured kernel package prevents a narrowly scoped audio-package transaction; no system packages/audio configuration were changed. The proposed pinned setup and limitation are documented in docs/p26-ubuntu-portability.md.

WINDOWS REVALIDATION IS PENDING. Keep this PR draft and do not merge as cross-platform-safe until actual launcher/default browser, zero console shows, second-launch runtime/session preservation, recording continuity/finalization, Restart Audio, Stop/Quit, owned descendant cleanup and unrelated-process protection pass on Windows. CI cannot substitute for that desktop/audio gate.

Historical Windows launchers, native ownership tests and screenshots are retained. See tasks/TASK-2.6.md, HANDOFF.md and docs/p26-validation.json. No TASK-2.4 was invented. P3 was not started.

Latest commit be2e730 passed both Ubuntu and Windows CI: https://github.com/astrobyte-dev/astros-beatbox/actions/runs/34072139609 . This includes shared browser journeys and native Windows ownership tests, but does not close the real Windows desktop/audio acceptance gate.