macOS desktop release ships without helpers, so KiCad IPC never starts #510

On the v0.13.0 darwin-arm64 desktop release, every KiCad IPC call fails:

KiCad IPC unavailable: the headless KiCad for <project> did not say it was listening within 30 s.

Cause: the macOS release ships no helpers. backplane-kipc, -step2glb, -voice and -browser are missing from the app's backplane-service folder and from the top of the release. The hub falls back to backplane-kipc on PATH, nothing spawns, and KServe.late reports the timeout. KiCad isn't at fault: kipc's socket polling handles bp.6's buffered stdout. 2655df8 fixes where the desktop hub looks for helpers, but on macOS there is nothing to find.

The same gap breaks Mech-tab previews of STEP parts that aren't cached yet (backplane-step2glb not found).

Workaround: put backplane-kipc on the hub's PATH (for example, link a dev build into ~/.local/bin), then restart the hub. ⌘Q leaves the resident hub running, so it has to be killed. IPC then reports ready with KiCad 10.99.0-bp.6.

Suggested fix:

• Stage the macOS helpers into the service payload the way build-windows-helpers.sh / prepare-windows-helpers.py do for Windows.
• Make the release producer reject a target that's missing a helper.
• Report "helper not found" instead of the KiCad timeout.

Environment: Backplane 0.13.0 (371f6d3), KiCad 10.99.0-bp.6 (8e4447d), macOS 26 arm64.