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/core/PlatformHooks.lua

@@ -0,0 +1,18 @@
+-- Generic process-lifecycle mod hooks so a platform-specific launcher
+-- integration (a native shell embedding this engine, e.g. wrapping the
+-- window in a platform UI) can live entirely in a mod instead of
+-- hand-patching main.lua, which every other engine change also touches.
+-- See docs/modding.md's "Process-lifecycle hooks" section.
+local ModRuntime = require("src.mods.Runtime")
+
+local PlatformHooks = {}
+
+function PlatformHooks.update(game, dt)
+  return ModRuntime.call("core.update", function(g, d) g:update(d) end, game, dt)
+end
+
+function PlatformHooks.quitToLauncher(vanilla)
+  return ModRuntime.call("core.quit_to_launcher", vanilla)
+end
+
+return PlatformHooks