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.

scripts/gen_lua_stubs.py

@@ -123,7 +123,7 @@ def render(modules):
             "-- Callbacks an app may define as globals:",
             "--   init()                   once, before the first draw (required)",
             "--   draw()                   every 33 ms while the app runs",
-            "--   on_tick()                every TICK_MS, when that global is set",
+            "--   on_tick()                at the interval app.setTickInterval() asked for",
             "--   on_touch_down(x, y)      finger down",
             "--   on_touch_up(x, y)        finger up",
             "--   on_touch(x, y)           tap, fired on release like a click",