Keys: which ones are held, not only which were pressed

Apps heard a key once, when it went down. A Game that moves something
for as long as a key is held (issue #101) asks the key mapper instead.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0162FokPdvY2KsS4NBfwyWPk

lib/input/src/key_mapper.h

@@ -33,6 +33,12 @@ class KeyMapper {
     // don't need Fn.
     void setTextEntry(bool active) { textEntry_ = active; }
 
+    // Is this key down right now, as of the last update()? Events only say when a key goes down:
+    // a Game that moves something for as long as a key is held asks this (issue #101). The arrows
+    // are ; . , / as for events: alone outside Text Entry, with Fn anywhere.
+    bool held(Key key) const;
+    bool heldChar(char c) const;  // a character key, as it types
+
     // 0 when idle, '*' when opt was pressed, or the accent character waiting for its letter.
     char pendingCompose() const { return compose_; }