feat(m188): launcher.log 早落盘 + 会话日志搬到 logs/game（旧家只读兼容）

用户投诉原话：「我们的启动日志放在哪？…他找半天没找到崩溃日志」「只有一个安装日志」。

启动器运行日志（.ULL/logs/launcher.log）
- presenter 构造 HostEnv 时就登记日志目录：启动早期缓冲的 200 行（[probe]/[font]/[theme]）
  当场刷盘，不再是「只有开过一局游戏才有日志」。
- app/** 里所有 eprintln! 诊断改走 app::diag::diag!（stderr + 文件双写，行首 [tag] 即 scope），
  含 boot/boot::fallback/state/update 各线。
- boot 那一拍补一行后端事实（request/effective/fallback）。
- 滚动上限（1 MiB → launcher.1.log，只留一份）写进 README 与 docs/logs-location.md。

会话日志搬家：data/agent/sessions → logs
- LauncherLayout: sessions_root()/game_logs() 新增，agent_sessions() 降级为只读兼容。
- host/logs 与 cli/commands/sessions 两个根合并（同 id 新根优先），event_path 回落旧家。
- 不搬不删；日志目录写一份 session-log-location.json 记录（moved=0, policy=read-only-compat）。

入口与文案
- 设置页「通用」新增「日志文件夹」行：路径 + 打开目录 + 复制路径（i18n 中英）。
- 日志页页头的「打开目录」不再是占位：走 M187 的 open_dir，失败文本带路径并送进剪贴板。
- README.md / README.zh-CN.md 新增「日志在哪」章节（含 crash-reports 只在游戏走到那一步才有），
  docs/logs-location.md 给出上限、双写判据、迁移策略与没验的部分。

守门（每条都有变异证明，见报告）
- 旧家会话仍列得出/打不开 ⇒ 只读新根变红；新会话写 logs/game ⇒ 只读旧根、写回旧家都变红。
- launcher.log 滚动 ⇒ 干掉 rename 变红。
- 打开日志文件夹指向真实目录 ⇒ 换成 cache/ 或去掉 create_dir_all 都变红。
- 日志根随三模式变化 ⇒ 把 HostEnv::logs_dir 写死成便携落点变红。

README.md

@@ -205,6 +205,31 @@ lists them all: `instances`, `install`, `launch`, `pack import`, `accounts`, `se
   `ull update apply`. Debug builds can rehearse the whole chain against a local directory source
   (`ULL_UPDATE_DIR`); release builds do not contain that path at all.
 
+### Where the logs are
+
+Everything the launcher writes on its own side lives under `<data root>/logs/`. The settings page has
+a **Log folder** row that shows that path with *Open folder* and *Copy path*; the logs page header has
+*Open folder* as well. The data root is `<exe folder>/.ULL` in portable mode, `<per-user app data>/ULL`
+in standard mode, and `<custom root>/.ULL` in custom mode — the same value the settings page displays
+as the launcher data directory.
+
+| Path | What it is |
+|---|---|
+| `logs/launcher.log` | The launcher's own runtime log: boot probe, font/theme decisions, backend choice, and every launch attempt (command line facts, spawn result, exit). Rolled over at 1 MiB into `logs/launcher.1.log`, exactly **one** old copy kept, so it cannot grow without bound. |
+| `logs/launcher.1.log` | The previous 1 MiB, replaced on the next rollover. |
+| `logs/game/<session-id>.jsonl` | One JSONL event stream per launch: the command line (secrets redacted), every stdout/stderr line, exit code. |
+| `logs/game/index.jsonl` | The one-line-per-session index the logs page lists. |
+| `logs/install.log` | Install pipeline journal, one line per stage. |
+| `logs/session-log-location.json` | A record of where session logs live and where they used to live. Rewritten on every start. |
+| `data/agent/sessions/game/…` | **Read-only compatibility**: sessions written by older builds. They are still listed and still openable; nothing is moved and nothing is deleted. |
+
+The game's own files are not in there — they sit in the instance's run directory: `logs/latest.log`,
+`crash-reports/crash-*.txt`, and `hs_err_pid*.log`. **`crash-reports/` only exists when the JVM lived long
+enough to write one.** If a mod, a missing library, or a wrong Java kills the game before Minecraft
+initialises, the game never gets that far, and the honest evidence is the session JSONL plus
+`launcher.log` — not a crash report. That is how the game behaves, not something the launcher is
+hiding.
+
 ### Not done yet
 
 This list is the reason to believe the rest. None of it is hidden behind marketing copy.
@@ -216,7 +241,7 @@ This list is the reason to believe the rest. None of it is hidden behind marketi
   real `releases/latest/download/manifest.json` was unreadable while `0.2.2` was still a pre-release.
   The first observation on the live stable channel happens after `0.2.3` ships.
 - **Several buttons are placeholders.** On the instances page: *New instance*, *Import*, *Duplicate*,
-  *Open folder*, *Delete*. On the logs page: *Open folder*. In the settings frame: *Save*, *Reset
+  *Open folder*, *Delete*. In the settings frame: *Save*, *Reset
   section*, *Add override*. Each writes `[page] 占位动作 …` to stderr and changes nothing — instance
   management from the GUI is the instance-wizard work still outstanding, and settings change as you
   edit them rather than on a Save.