Tools wave 2 — blog, migration guide, theme builder

Three more Koder Design tools land in a single session, taking the umbrella #049 (Material.io parity) tools drain from 1/5 to 4/5: `/blog/` (release log driven by `releases.toml`), `/migrate/material/m3/` (hand-curated Material 3 → Koder migration guide with 19-row token mapping + 15-row component mapping + stepwise upgrade), and `/themer/` (interactive seed → CSS-vars export Theme Builder MVP with light/dark + URL-hash persistence). Only `/figma-export/` remains, parked as a v2 spec because it needs a Figma plugin runtime rather than a static page.