test: the servers two_user starts get a Windows environment (P11c)

two_user.rs started its flagged servers with env_clear plus a Unix PATH
and HOME, no SystemRoot: on Windows Winsock cannot start without it, so
the --http door failed to bind (the server says so on stderr and serves
its pipes only, which is why "ready" still came). The tests now use the
harness's clean_env, as do evidence.rs's extract and the viewer's
command_line test. main (a8ce2e7d) merged in.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

crates/ledger-gui/tests/command_line.rs

@@ -10,6 +10,13 @@ use std::process::Command;
 fn viewer(home: &std::path::Path) -> Command {
     let mut c = Command::new(env!("CARGO_BIN_EXE_ledger-gui"));
     c.env_clear().env("PATH", "/usr/bin:/bin").env("HOME", home);
+    // A Windows program started with no environment at all can't load.
+    #[cfg(windows)]
+    for name in ["SystemRoot", "windir"] {
+        if let Some(value) = std::env::var_os(name) {
+            c.env(name, value);
+        }
+    }
     c
 }