refactor: drop dead resolveGamescopePid, use helper MainPID directly (#37)

[hikaps]

Removes a spurious, actively-misleading warning that surfaced in issue-28 logs (Instance 0 could not resolve gamescope PID from helper PID …) and looked like a culprit for a missing second Steam instance.

Root cause: dead logic, not a real failure

LaunchInstance returns the gamescope MainPID directly — the transient unit's ExecStart is gamescope (systemd-run … -- gamescope <args> -- /bin/bash -c <game>, Type=simple). But resolveGamescopePid() assumed the returned PID was a wrapper whose child is gamescope: it read /proc/<pid>/children and matched comm == gamescope. Gamescope's actual children are bash→the game, so it always returned 0 and logged the warning. The fallback on the next line already set m_gamescopePid = m_helperPid, i.e. the correct value.

So m_gamescopePid, WindowManager positioning, and SessionRunner::findSteamProcess() were all using the correct PID — the only effect was the false log line.

Change

Delete resolveGamescopePid (1 call site, 0 test refs) and assign directly:

// The helper's LaunchInstance returns the gamescope MainPID directly.
m_gamescopePid = m_helperPid;

Behavior unchanged (the fallback already produced this value); −37 lines.

Verified

test_gamescopeinstance: 21 passed, 0 failed.

Note: this does not fix the flatpak second-Steam-instance issue reported in #28 — that's flatpak-specific (host Steam coexisted fine in the native path), tracked separately.