Introduces src/platform.rs with OS detection and env var overrides.
All hardcoded Linux paths replaced with Platform::detect() across
8 source files. Key changes:
- New Platform struct with 11 fields (all overridable via env vars)
- /proc/ access gated to Linux (#[cfg(target_os = "linux")])
- pgrep/pkill patterns broadened for cross-platform LS discovery
- sec-ch-ua-platform header now dynamic per OS
- Token, traces, config, CA cert paths use platform module
- LD_PRELOAD DNS redirect gated to Linux only
- Setup scripts for Linux (systemd) and macOS (launchd)
- find_ls_binary_path has cross-platform stubs
All 46 tests pass, cargo check clean.
