STORY-003: Fix stop button + canvas delete (upstream superdesign #80)

[qte77]

Goal\nFix the stop-generation button and canvas element delete so both work reliably without race conditions.\n\n## Upstream reference\nsuperdesigndev/superdesign#80\n\n## Acceptance criteria\n- Stop button cancels in-flight generation\n- Canvas delete removes element without state corruption\n- Unit tests for both flows (RED then GREEN)\n- make validate passes before merge\n\n## TDD gate\nFollow .claude/rules/tdd-enforcement.md.