feat(tailwind): add middle gradient stops

[doodlewind]

Summary

• compile via-* Tailwind utilities into an optional 50% gradient stop
• resolve three-stop gradients inside the engine core into the existing DrawList operations
• preserve clipping, transformed painter order, rounded coverage spans, and 3D flattening
• document and contract-test the new styling surface

Validation

• bun run test (11/11 stages green)
• bun test tests/tailwind.test.ts
• bun tests/contract.ts
• cargo test --manifest-path engine/core/Cargo.toml (124 passed)
• git diff --check

Architecture

No DrawList opcode or backend contract changes. The engine core lowers a middle stop into the existing GRAD_RECT/TRI primitives; rounded boxes continue to use alpha-covered RECT spans.

Known baseline

cargo clippy --manifest-path engine/core/Cargo.toml -- -D warnings reports existing Rust 1.96 lint debt in unchanged files/functions; the complete test suite and core tests pass.