Add solid start-region shapes: SOLID_RECT and SOLID_OVAL

[PiotrPrus]

Summary

Adds filled start-region shapes so particles can spawn from anywhere inside the area, not just the outline. Closes #24.

• SOLID_RECT — random point uniformly inside the width × height box.
• SOLID_OVAL — random point uniformly over the ellipse's area, using the sqrt(random()) radius trick so it isn't center-biased.

Existing OVAL/RECT (outline) behavior is unchanged; the solid variants are additive and non-breaking. isInsideStartRegion (used by hideInStartRegion) now handles both solid shapes, reusing the interior tests of their outline counterparts.

Sample

New Disintegrate sample demonstrating the feature: tap a solid blue shape to blow it apart into particles that fall under gravity.

• Shape toggle — SOLID_RECT ↔ SOLID_OVAL (rendered as a circle).
• Edge-behavior toggle — None ↔ Bounce(damping = 0.3).
• Particle-count slider (50–1500) controlling the burst size.
• Single state-driven button: 💥 Boom → ↻ Recreate.

Docs

• KDoc on the Shape enum entries, startPoint, and startRegionSize.
• README: features list, a new Start Region Shapes table covering all shapes, and an updated hideInStartRegion note.

Testing

• :particle-emitter:compileKotlinMetadata and :samples:shared:compileCommonMainKotlinMetadata compile cleanly.
• Verified live on a Pixel 9a emulator: both solid shapes emit from the whole filled area, the count slider changes burst density, and Bounce(0.3) makes particles settle along the bottom edge.

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