Add visible character control to Text and BitmapText

[obiot]

Summary

Add properties to control how many characters of a text renderable are visible, enabling typewriter effects, progressive text reveal, and dialogue systems.

Proposed API

// character count — show only the first N characters (-1 = all)
text.visibleCharacters = 5; // shows "Hello" from "Hello world!"
text.visibleCharacters = -1; // shows all (default)

// ratio — percentage of text visible (0.0 to 1.0)
text.visibleRatio = 0.5; // shows first 50% of characters
text.visibleRatio = 1.0; // shows all (default)

Setting visibleRatio auto-syncs visibleCharacters based on total character count, and vice versa.

Typewriter effect with Tween

const dialog = new BitmapText(10, 10, {
    font: "PressStart2P",
    text: "The princess is in another castle..."
});
dialog.visibleRatio = 0;

// animate text reveal over 2 seconds
new Tween(dialog).to({ visibleRatio: 1.0 }, 2000).start();

Implementation Notes

• BitmapText: skip glyph rendering past visibleCharacters count in the draw loop
• Text: substring the text before canvas fillText/strokeText calls
• Both properties should work together — setting one updates the other
• visibleCharacters = -1 and visibleRatio = 1.0 should be the defaults (full text visible, zero overhead)
• Should work with multi-line text (count includes characters across all lines, excluding line breaks or including them — TBD)