ci: probe FileProvider root from signed HostApp

[Jesssullivan]

Summary

• add a signed HostApp root probe gated by TCFS_FILEPROVIDER_ROOT_PROBE=1
• have the macOS postinstall smoke invoke that probe when shell/coordinated CloudStorage root enumeration fails
• if the signed HostApp can enumerate root entries, continue far enough to drive signed-host requestDownload before shell path checks

Verification

• bash -n scripts/macos-postinstall-smoke.sh scripts/test-macos-postinstall-smoke.sh
• git diff --check
• env -u DEVELOPER_DIR -u SDKROOT -u NIX_CFLAGS_COMPILE -u NIX_LDFLAGS -u NIX_CC /usr/bin/xcrun --sdk macosx swiftc -parse-as-library -typecheck swift/fileprovider/Sources/HostApp/HostApp.swift
• bash scripts/test-macos-postinstall-smoke.sh

Tracking

• TIN-1547
• GitHub Prove distribution install and upgrade flows across supported release surfaces #280