[tldraw] Add option for geo shapes to shrink text instead of expanding bounds with growY

## Problem When text inside a geo shape exceeds the shape's bounds, the shape automatically grows vertically via `growY` to accommodate the text. This is the only available strategy — there's no way to keep the shape at a fixed size and instead reduce the font size to fit the text within the existing bounds. This matters for use cases where maintaining consistent shape dimensions is important, such as: - Diagram layouts where shapes must stay a fixed size - Flowcharts with uniform node sizing - UI mockups with constrained containers - Any design where spatial consistency takes priority over text readability at full size ## Proposed solution Add an option to geo shapes (and potentially other label-bearing shapes) that controls the text overflow strategy. When enabled, instead of expanding the shape with `growY`, the shape would progressively reduce the label font size until the text fits within the shape's current dimensions. Possible approaches: 1. **A shape-level prop** (e.g.,