[KS] API: privileged helper, admin prompt, XPC (#138)

[sethwebster]

Refs #138

Adds @legendapp/spark/privileged-helper (macOS): a launch daemon the app ships, registered through SMAppService with its Login Items approval states; the system admin prompt (Authorization Services), whose rights travel to the helper, which checks them itself; and JSON over XPC with each side checking the other's code signature. Unsigned/ad-hoc builds report unsigned and stop before ServiceManagement. macos.privilegedHelpers opts helpers in; spark build writes their LaunchDaemons plists, nothing otherwise. Windows is typed unsupported (#440). Guide and threat model: docs/privileged-helper.md.

Security details:

• The helper accepts only the app: team + app identifier, plus a Developer ID leaf when the helper is Developer ID signed; on connect it reads the client's signature by audit token and refuses clients without the hardened runtime or with get-task-allow.
• The app requires the helper's team and its signing identifier (its launchd label; spark build/spark package sign it so, and spark package checks it). The app sends nothing until the helper's verification reply passes; interrupted connections are marked stale synchronously and verified again.
• Admin rights: AdminAuthorization.expiresAt reports when system.privilege.admin's timeout lapses them (300 s); lapsed rights reject authorizationExpired (app-side, and helper-side before the handler) instead of a silent authorized: false. The helper checks with flags 0 (no interaction, no extending).

Kitchen Sink: Processes and IPC > Privileged helper, PROC checks, driver flows, a manual real-path flow (team-signed build with examples/kitchen-sink/helpers/privileged-helper), goldens.

Verification (run in this session, after rebasing onto the current base)

• bun run typecheck: passes
• bun run test: 1974 passed, 2 failed (accepted: node-runtime asdf shim; motion-native frames check)
• tests/privileged-helper-native.test.ts: real XPC round trips against an anonymous listener in the test process (test binary signed ad hoc with the hardened runtime): both signature checks, the hardened-runtime/get-task-allow check (a non-hardened build of the binary fails it), a lapsed authorization rejected before the handler, helper errors, timeout, abort; the simulation; a build without the driver; the Kitchen Sink helper compiling. tests/packaging.test.ts: plists only on opt-in, helpers signed with their labels.
• bun run e2e:lint: 0 problems
• Driver flows under ks-lock native: all e2e/flows/processes-ipc macOS flows pass (Windows skipped, manual awaiting sign-off), accessibility/audit-processes-ipc (4) and infra/catalog-driver (4) pass; the catalog still reads 26 of 38 areas, so its goldens did not change after the rebase
• security/entitlements.yaml (SEC-ENT-01) passes on a release build of this branch; it has no Contents/Library
• Only covered by the manual flow (privileged-helper-real.yaml, needs Seth on a Developer ID build): the root-side authorization check with a granted right, the real admin prompt, SMAppService registration/approval, rights lapsing after 300 s, and the Developer ID client requirement. Automated runs never reach a granted right.
• Not run: Windows

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