fix: complete post-migration corrections and asset optimization

[flsobral]

What changed

• restored favicon/manifest and accessible static social icon behavior;
• kept SPDX metadata verifiable in source without rendering it into pages;
• routed large raster media through responsive Astro AVIF/WebP generation;
• replaced the 8.3 MB GIF with WebM/MP4 plus a WebP poster;
• added deterministic asset reports, hard budgets, CI/Pages enforcement, and an asset-optimization skill;
• prepared the v1.0.1 patch release.

Why

The Astro migration preserved routes and layout but left several identity regressions and delivered large source rasters/GIFs directly. This patch restores the missing behavior and prevents payload regressions at generated-output boundaries.

Impact

The homepage fallback-resource proxy falls from 16.34 MiB to 2.01 MiB, get-started falls from 9.07 MiB to 1.57 MiB, and the animated demonstration falls from 8.3 MB to 461 KB across preferred/fallback/poster formats. Routes, content semantics, licenses, and the established design remain unchanged.

Validation

• clean-clone npm ci with zero vulnerabilities;
• 17 repository tests passed;
• license policy 174/174;
• release and workflow policy checks passed;
• Astro diagnostics: 0 errors/warnings/hints;
• 18 pages built;
• asset budgets: 0 failures, 2 warnings below the 1 MiB ceiling;
• route/link/SPDX/RSS/sitemap/content validation passed;
• automated desktop/mobile screenshots and headless video playback evidence captured.