feat: Width and Height, an object's drawn size

[LucasAntunesdeAlmeida]

What

o.Width() float64 and o.Height() float64: the size an object is drawn and clicked in.

• Size when set; else a Rect's size, a sprite's image size (once the object is in a scene), or a text's measured size: its fonts with fallbacks, TextSize, the Wrap column, lines LineHeight apart, plus a button's plate margins.
• Never the Hitbox, which only affects collisions.
• Text is re-measured on every change already (measureText runs in Text, SetText, Font, TextSize, Wrap, LineHeight), so both getters are current in the same frame, headless and in go test, with no window and nothing drawn.

Games no longer have to guess len(s)*size, which is wrong for multi-byte and fallback-font text. They can lay out labels by their edges and test that every translation fits its box.

Design choices

• Name. .Size(w, h) is already the setter. Width()/Height() match the existing g.Width()/g.Height(), and a single value can be used inline: label.X = left + label.Width()/2. A (w, h) pair can't be used that way.
• Drawn size (bounds()), not the collision box(). Layout and fit checks need what the player sees. Agents keep getting the hitbox in observations.
• No package-level Measure(s) helper. engine.Text(s).Font(...).TextSize(n).Wrap(w).Height() already measures a string with no game or scene. A text object is cheap, and this form takes exactly the same styling calls the real label uses. A separate helper would copy that styling as parameters.
• Restart fix. restoreState now re-measures a restored text with the font, size and wrap it has now. Before, a text restyled after setup came back with its old box and was drawn in it. The new test fails without this fix.

Proof

• size_test.go:
  • Rect, Size and Hitbox, and a sized text area.
  • Latin, then Cyrillic after SetText (measured per glyph, not per byte), TextSize, Font, and a button.
  • CJK through testdata/fallback.ttf: Latin-only text, all-CJK text, and a mixed line.
  • Wrapped CJK text (column x 3 lines), LineHeight(40), and "\n" lines.
  • Headless: SetText then Width inside OnUpdate, anchoring by the left edge in that same frame, checked against the observation, then Restart.
  • A sprite before and after Add, with and without Size.
• examples/dialog: the speaker's name is placed at the speech's left edge using Width after each SetText, which replaces the Wrap-column workaround. The CLICK hint's right edge is placed at x 770. Over MCP, English lays out pixel-identically to before (ELDER at x 250 w 120, CLICK at 740 w 60), so demo.gif stays valid. zh-CN gives 长老 with its left edge at 190 and 点击 with its right edge at 770.
• README: cheat-sheet row, Dialog row, and contract item 13 ("Text is measured the moment it changes").
• Full local gate passes: gofmt, vet, staticcheck, test, plus js/wasm and windows vet+build.