Decode a system Back intent

handle("system_back", "app", "intent:canGoBack")

Native Back carries a positive intent ID plus a 1 or 0 host-history hint; the application must answer that exact intent once.

• Close app-owned transient UI such as a modal before changing navigation.
• Use the parsed positive intent ID, never a locally invented number.
• A stale or repeated intent is rejected by the host.
• systemBackExit is host-dependent and must remain the final root-level decision.

Focused example

local intentText, canGoBackText = string.match(value, "^([1-9][0-9]*):([01])$")
local intent = if intentText == nil then nil else tonumber(intentText)

if action == "system_back" and target == "app" and intent ~= nil then
    if modalOpen then
        modalOpen = false
        Host.systemBackHandled(intent)
    elseif router.canBack() then
        Host.systemBackHistory(intent)
    elseif canGoBackText == "0" then
        Host.systemBackExit(intent)
    else
        Host.systemBackHistory(intent)
    end
end

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