messagix/bloks: support bk.action.core.Let

[iFixRobots]

Adds the missing bk.action.core.Let opcode. Without it, the Messenger Lite login flow fails on the email/password screen with tapping login button: on_touch_down: unimplemented function bk.action.core.Let (2 args) — Facebook's login script started using Let to lazy-init per-screen animation state.

Let(currentOrNull, FuncConst(initializer)): if currentOrNull evaluates to a non-null value (a previously cached value), return it; otherwise apply the FuncConst lambda in arg 1 and return its result. Implementation matches the existing Coalesce pattern plus the Apply lambda invocation.

Reproduced and verified against a real captured bundle: the on_touch_down handler now executes through to com.bloks.www.bloks.caa.login.async.send_login_request cleanly via pkg/messagix/bloks/cmd -file <bundle> -action -login.

Closes PLAT-36280.

[radon-at-beeper]

Could also implement this as a macroexpansion since the code as written is basically saying "do bk.action.bool.Or then bk.action.core.Apply". Example:

meta/pkg/messagix/bloks/interp.go

Lines 800 to 815 in cd0293c

	case "h9a":
	// ignore second argument for now, use first & third
	return i.Evaluate(ctx, &BloksScriptNode{
	BloksScriptNodeContent: &BloksScriptFuncall{
	Function: "bk.action.core.Apply",
	Args: []BloksScriptNode{
	call.Args[2],
	{
	BloksScriptNodeContent: &BloksScriptFuncall{
	Function: "bk.action.string.EncryptPassword",
	Args: []BloksScriptNode{call.Args[0]},
	},
	},
	},
	},
	})

But that's not really necessary, so approving as is.