feat(shutdown): passive drain and rename DisableSoftCrash

Shutdown now proceeds in three phases: close listeners immediately on
signal, broadcast the in-game countdown, wait up to ShutdownDrainSeconds
(default 30) for sessions to disconnect naturally via DrainPassive, then
force-close any stragglers. This prevents players from entering new
quests after the countdown starts, and lets mid-quest sessions finish
saving without being killed mid-write. A second SIGINT during passive
drain cancels it so the force-close phase runs immediately.

Renamed DisableSoftCrash -> DisableShutdownCountdown since the flag
controls the countdown, not crash behaviour. Existing config.json files
keep working via a Viper alias on the legacy key.

Closes #179.

CHANGELOG.md

@@ -7,6 +7,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Shutdown now proceeds in three phases: listeners close immediately on signal, then a passive drain waits up to `ShutdownDrainSeconds` (default 30) for sessions to disconnect naturally, then remaining connections are force-closed. This prevents players from starting new quests after the countdown begins ([#179](https://github.com/Mezeporta/Erupe/issues/179)).
+- Renamed config field `DisableSoftCrash` → `DisableShutdownCountdown` for clarity. The old key is still accepted via a Viper alias so existing `config.json` files keep working without modification ([#179](https://github.com/Mezeporta/Erupe/issues/179)).
+
 ### Added
 
 - Reverse-engineered user binary data types from `mhfo-hd.dll` via Ghidra: type 1 = character name (max 17B SJIS), type 2 = player profile with self-introduction (208B), type 3 = equipment/appearance snapshot (384B). Added structured parsing with size validation warnings to `handleMsgSysSetUserBinary`.