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.

docs/lua-api-parity.md

@@ -12,7 +12,7 @@ Compared against crosspoint-reader at `src/util/lua/LuaBindings*.cpp`.
 `fs.listDirs`, `fs.listFiles`, `fs.exists`, `fs.readFile`, `fs.writeFile`,
 `gui.width`, `gui.height`, `gui.fillRect`, `gui.drawRect`, `gui.drawLine`,
 `sys.millis`, `sys.delay`, `sys.exit`, `log.debug/info/error`, the whole `http` table,
-and the `init()` / `draw()` / `on_tick()` callbacks. `init()` is required in both, so a
+`app.setTickInterval`, and the `init()` / `draw()` / `on_tick()` callbacks. `init()` is required in both, so a
 misspelled entry point is an error rather than an app that quietly draws nothing.
 
 ## Deliberate differences
@@ -38,7 +38,6 @@ Each of these is the same concept spelled two ways. Fixing them means changing o
 
 | Concern | crosspoint-reader | esp32-lcd | Suggested resolution |
 |---|---|---|---|
-| Tick interval | `app.setTickInterval(ms)` | global `TICK_MS` | Pick one mechanism. |
 | `wifi.status()` | returns a **string** | returns a **table** of state/ssid/ip/rssi | Same name, incompatible types — the sharpest edge here. This repo added `isConnected()` and `localIP()` so the crosspoint idioms work either way. |
 | `wifi.connect()` | no arguments, uses stored credentials | `(ssid, password)`, saves them | Both are wanted: a no-argument reconnect and an explicit join. |
 | `fs.readFile` cap | 50000 bytes | 65536 bytes | Arbitrary in both. |