refactor: replace the TICK_MS global with app.setTickInterval()

A magic global that the runtime reads once at startup could not be changed later, gave
no feedback when misspelled, and was a second spelling of a mechanism the sibling
firmware already had. app.setTickInterval(ms) clamps to 33..3600000, takes 0 to stop,
and errors when on_tick() is not defined -- by the time init() runs the chunk body has
finished, so a missing callback is a typo rather than a race.

Also drops the code comments pointing at the other repo. Where the two APIs agree or
differ belongs in docs/lua-api-parity.md; a comment beside a constant explaining that
another firmware picked the same number is noise a reader here cannot act on.

test/fake_device.lua

@@ -13,6 +13,7 @@ local device = {
   rotation = 0,
   theme = "light",
   clockSynced = false,
+  tickInterval = 0,
   timezone = "UTC0",
   raw = nil,          -- pending raw touch reading, {x, y} or nil
   networks = {},      -- what wifi.scan() returns
@@ -99,7 +100,14 @@ function device.install()
     forget = function() device.connected = nil return saved() end,
   }
 
-  log = {info = function() end}
+  app = {
+    setTickInterval = function(ms)
+      assert(ms == 0 or on_tick, "setTickInterval without on_tick")
+      device.tickInterval = ms
+    end,
+  }
+
+  log = {debug = function() end, info = function() end, error = function() end}
 
   return device
 end