Add declarative rect gradient fills

[han4wluc]

Summary

• add declarative rect fill support for solid, linear-gradient, and radial-gradient schema shapes
• map structured rect fills to Pixi FillGradient at runtime and clean up gradient resources on update/delete
• add docs, parser/runtime tests, a VT spec, its accepted reference, and bump the package version to 1.0.4

Verification

• bun run lint
• bun run test
• docker run --rm --user $(id -u):$(id -g) -e RTGL_VT_DEBUG=true -v "$PWD:/workspace" docker.io/han4wluc/rtgl:playwright-v1.57.0-rtgl-v1.0.10 rtgl vt report