refactor: split Linux system service adapter

[turinglambdaai]

Summary

• split the 1,463-line Linux system adapter into focused single-instance, notifications, tray, autostart, secure-storage, and crash/shutdown implementation units
• keep XDG paths, the shared D-Bus connection, error translation, and low-level writes in one private support core
• update every first-party Linux host target and document the resulting reader-facing boundary; the public system_services.hpp API is unchanged

Advances #185.

Validation

• raco test tests/system-services.rkt tests/codegen.rkt tests/linux-installer.rkt tests/linux-native-installer.rkt — 199 tests passed
• git diff --check
• Linux compiler and embedded-runtime validation run in CI

[turinglambdaai]

Merged current main after #198/#199. The public system-services surface remains unchanged; re-ran system service, codegen, Linux installer, and native installer suites: 199 tests passed; diff check is clean.