fix: remove nested split-button trigger wrapper

[zeno-codebot]

Summary

• remove the split-button <details><summary> wrapper so the menu trigger is no longer nested inside another interactive element
• render the secondary trigger as a standalone cindor-button inside a non-interactive .menu-shell while preserving menu state, ARIA wiring, and Escape focus restoration
• update split-button tests to target the standalone trigger and assert that no summary or details element is rendered

Verification

• npm run test -- --run packages/core/src/components/split-button/cindor-split-button.test.ts
• npm exec -- eslint packages/core/src/components/split-button/cindor-split-button.ts packages/core/src/components/split-button/cindor-split-button.test.ts

Notes

• scoped to split-button and directly related tests only