bug(composition): style values admit raw-text breakouts and CSS escape bypasses #448

Security review findings (medium CWE-79 + low CWE-918).

assertVisualStyleV1 in packages/composition/src/visual.ts blocks literal url(/image-set(/var(/env(/attr( and declaration separators but allows < and \:

• Animation keyframe values are emitted verbatim into a raw-text <style> element (packages/html-program/src/document.ts), so red</style><script>… terminates it and injects markup into the render document.
• Browsers resolve CSS backslash escapes before recognizing functions, so u\72 l(https://evil/) passes the literal url( check and decodes to a resource fetch — an SSRF bypass of the typed-Artifact policy.

Proposed fix: reject < and \ in style values — parity with assertColor, which already rejects both.

Fix: #440