fix: keep user control available during boot activation

[PLN]

An enabled notify unit previously hid the user control endpoint for its entire startup wait. A workload querying its own control pipe before READY could not complete boot. Open and serve user control before default/graphical-session target activation, matching the SYSTEM manager. Listener failure cancels pending boot work.

Validation:

• The native regression reproduces the missing endpoint before this change. With the fix it reads activating status and stops a real pending workload. A second regression verifies that an occupied control pipe prevents enabled workload launch. Both passed five race-enabled repetitions; the full local race-enabled suite passed.
• Exact-source CI 34817416447 passed for fdf9d2f. The immutable Windows artifact, source/module identity and dependency/artifact hashes were verified.
• Disposable Enterprise LTSC build 26100, real SYSTEM broker and two genuine standard-user S4U managers: enabled notify workloads successfully queried their own control endpoint before READY during manager replacement. The equivalent fixture could not complete that step with the preceding source.
• Each replacement also booted a separate notify workload that never sent READY. Both workloads reached their own control pipe, and external status identified the correct native PID/invocation as activating. Stop completed in 0.032s and 0.028s; each process exited and its unit became inactive with no uncertain termination.
• Five notifying and two pending workload invocations, 41 ordered ownership snapshots, same-name SYSTEM/user notification isolation, stale endpoint removal, peer stability, independent revocation and no resurrection passed. Cleanup removed pending unit/enablement records, fixture processes, temporary account/profile and linger grants; all five filesystem ACLs and the original broker were restored.

This fixes user control availability during boot and qualifies the stated headless launch/stop cases. The remaining real interactive/session matrix stays open. Raw evidence is retained privately.