Let On key down repeat while the key stays held

A held key fired once and then nothing, so sliding a Tetris piece sideways or
walking a menu meant building a timer next to every key node. Two settings on
the node do it now: Repeat after and Repeat interval, both in milliseconds so
the usual values stay whole numbers (300, 50) instead of 0.3 and 0.05. A delay
of 0 keeps the node exactly as it was, an interval of 0 means every frame.

The browser's own auto-repeat is deliberately not used - Input.press has always
dropped it - because its delay and rate are an operating-system setting of the
player's, not a game's. The repeat is built from the measured hold time instead,
per node, so two nodes on one key keep their own clocks. The rule itself is a
pure function in keyRepeat.ts and is checked without a world.

Both rows stay off the node body while the delay is 0: two lines of "0" on every
key node in every graph would be noise, and the screenshot said so. NodeDef gains
hiddenParams for that; the inspector still shows them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -102,6 +102,12 @@ shown on the node and editable in the inspector.
   (inspector → Parent, or drag it onto that object in the hierarchy; dashed line in the
   viewport). The `Owner` node returns it; `Self` returns the behavior itself; `Parent` walks up
   from any object.
+- **Auto-repeat on `On key down`**: two settings on the node, *Repeat after (ms)* and *Repeat
+  interval (ms)* – milliseconds, so both stay whole numbers. A delay of 0 leaves the node as it was – one fire per press; with a delay set, a
+  key that stays held fires again after it and then every interval (0 = every frame). That is the
+  sideways slide of a Tetris piece, or a held arrow walking a menu. The browser's own key repeat is
+  deliberately not used: its delay and rate are an operating-system setting, not a game's. Both
+  rows stay off the node body while the delay is 0, so a plain key node keeps its single line.
 - **Node set** (`src/core/graph/nodes/`, one file per node): events `On start`, `On update`, `On key down/up/held`,
   `On timer`, `On collision enter/stay/exit`, `On click` (the mouse pressed on the owner – anywhere when it has none), `Start` · flow `Branch`, `Sequence`, `Repeat`, `Run behavior`, `End` · data `Self`, `Owner`, `Parent`, `Object by name`, `Name of`,
   `Is asset` (is this object an instance of asset X – how you tell what you collided with), `Get/Set