feat: implement ecosystem contract and prepare v1.6.0

[Foscat]

Summary

• implement the reviewed ecosystem ownership, semantic token, compatibility, documentation, and release contracts
• prepare Interactive Surface CSS 1.6.0 with 8 retained exports and no removed exports
• pin the immutable UI Style Kit compatibility fixture and published Layout Style CSS 3.0.1 merge
• set the npm homepage to the live GitHub Pages demo: https://foscat.github.io/Interactive-Surface-CSS/

Verification

• TDD RED/GREEN/mutation checks for release metadata and fixture pinning
• 44/44 contract tests and 6/6 package tests
• 279/279 Playwright cases across Chromium, Firefox, and WebKit
• current and minimum seven-scenario ecosystem matrices
• CSS lint, generated parity, ownership, clean installs, and workflow policy
• npm pack/publish dry-run: 26 files, 8 exports
• npm audit and npm audit --omit=dev: 0 vulnerabilities

Compatibility

• minimum: ui-style-kit-css 2.1.0, interactive-surface-css 1.5.0, layout-style-css 3.0.0
• current: ui-style-kit-css 2.1.0, interactive-surface-css 1.6.0, layout-style-css 3.0.1
• SemVer impact: minor release; no export removals or major-version break