Add core.update/core.quit_to_launcher platform lifecycle hooks (RFC 0006)

A platform-specific launcher wrapper (a native shell embedding this engine,
owning its own UI around the game window) needs to pause the simulation
while its own UI is on top, live-reload options it wrote outside any Lua
UI, and veto main.lua's "closing the window returns to the Lua launcher"
behavior when it owns that job itself. Implementing this by hand-patching
main.lua's love.update/love.quit directly ties every such integration to
editing the one file every other engine change also touches, guaranteeing
merge conflicts. No existing hook covers "should the per-frame simulation
step run" or "should closing the window return to the Lua launcher."

Adds two generic, additive hooks (src/core/PlatformHooks.lua): core.update
and core.quit_to_launcher, replacing what would otherwise be inline
main.lua special-casing. Also adds Manifest.force_enable_env, letting a
mod that cannot function disabled on the one build where its env var is
set (a platform-bridge mod bundled only with that build) re-enable itself
regardless of a saved disable.

RFC 0006 status: Proposed.

src/mods/Loader.lua

@@ -951,6 +951,16 @@ function Loader:load(data)
       end
     end
   end
+  -- A manifest may name an env var that force-enables it regardless of a
+  -- saved disable in options.mods -- generic, not tied to any mod id, for
+  -- a mod (e.g. a native-launcher bridge) that cannot function disabled on
+  -- the one build where its env var is set.
+  for id, mod in pairs(self.mods) do
+    local envName = mod.manifest.force_enable_env
+    if envName and os.getenv(envName) == "1" then
+      self.disabled[id] = nil
+    end
+  end
   for id, mod in pairs(self.mods) do
     mod.enabled = not self.disabled[id]
     mod.state = mod.enabled and "pending" or "disabled"