fix(mods): stop the Gen 2 require facade answering the engine

bootGame's require("src.core.Game") was resolving to Gen2Compat's Game
facade after a Gold session, so the next Gen 1 boot got an empty proxy
whose rawget(load) is nil (iOS: "src.core.Game missing load after
reload"; Android: Game:load called on a nil value).

Two independent causes, both needed:

  * callerIsMod decided "not under src/" meant "a mod", and main.lua is
    not under src/, so the engine's own require was gated as a mod's.
    Root chunks main.lua and conf.lua are now matched exactly, which a
    mod's own mods/<id>/main.lua cannot collide with.

  * devShim.generation was only ever set, never cleared, so the
    generation Gold declared outlived Gold's session. Loader.endSession
    drops it from SessionLifecycle.endMountedSession, and the facade
    gate now requires generation == 2 rather than ~= 1.

Only platforms that return to the launcher in-process (Android, and iOS
since #1808) keep the shim alive across sessions; everywhere else the
process restart cleared it.

tests/engine/gen2_shim_engine_require.lua

@@ -0,0 +1,90 @@
+package.path = "./?.lua;./?/init.lua;" .. package.path
+
+love = love or require("tests.love_stub")
+
+local S = require("tests.harness").suite("gen2 shim engine require")
+local check, eq = S.check, S.eq
+
+local Loader = require("src.mods.Loader")
+local Gen2Compat = require("src.mods.Gen2Compat")
+local GameVersion = require("src.core.GameVersion")
+local SessionLifecycle = require("src.core.SessionLifecycle")
+
+local NAME = "src.core.Game"
+local REAL = { load = function() end }
+
+local function memfs(files)
+  return {
+    files = files,
+    read = function(path) return files[path] end,
+    write = function(path, content) files[path] = content return true end,
+    remove = function(path) files[path] = nil return true end,
+    getInfo = function(path)
+      if files[path] then return { type = "file" } end
+      local prefix = path .. "/"
+      for key in pairs(files) do
+        if key:sub(1, #prefix) == prefix then return { type = "directory" } end
+      end
+      return nil
+    end,
+    load = function(path)
+      if not files[path] then return nil, "no file: " .. path end
+      return load(files[path], path)
+    end,
+    createDirectory = function() return true end,
+    getDirectoryItems = function(path)
+      local seen, items = {}, {}
+      local prefix = path .. "/"
+      for key in pairs(files) do
+        if key:sub(1, #prefix) == prefix then
+          local child = key:sub(#prefix + 1):match("^[^/]+")
+          if child and not seen[child] then
+            seen[child] = true
+            items[#items + 1] = child
+          end
+        end
+      end
+      table.sort(items)
+      return items
+    end,
+  }
+end
+
+local function requireFrom(source)
+  local chunk = assert(loadstring(
+    "local name = ... local mod = require(name) return mod", "@" .. source))
+  return chunk(NAME)
+end
+
+local savedVersion = GameVersion.get()
+local savedModule = package.loaded[NAME]
+package.loaded[NAME] = REAL
+
+GameVersion.set("gold")
+local files = {
+  ["mods/facade/manifest.json"] =
+    [[{"id":"facade","name":"facade","version":"1.0.0","entry":"main.lua",]]
+    .. [["gen2compat":true}]],
+  ["mods/facade/main.lua"] = "return function(mod) end",
+}
+local loader = Loader.new({ fs = memfs(files) })
+loader:load({ pokemon = {} })
+
+eq(loader.generation, 2, "the fixture loader is a Gen 2 one")
+check(Gen2Compat.serves(NAME), "the facade serves " .. NAME)
+
+local facade = Gen2Compat.resolve(NAME)
+eq(requireFrom("mods/facade/main.lua"), facade,
+  "a mod's require still resolves to the Gen 2 facade")
+eq(requireFrom("main.lua"), REAL,
+  "main.lua's require is the engine's own, never the facade")
+eq(requireFrom("conf.lua"), REAL, "conf.lua's require is the engine's own")
+
+SessionLifecycle.endMountedSession()
+eq(requireFrom("mods/facade/main.lua"), REAL,
+  "after the session ends the facade stops answering for the next boot")
+
+package.loaded[NAME] = savedModule
+GameVersion.set(savedVersion)
+
+S.finish()