feat(blog): hydrate posts from Tinyland broker stream

[Jesssullivan]

Summary

• hydrate the canonical /blog index from the hub.tinyland.dev broker stream at runtime on CF Pages
• hydrate /blog/[slug] from reviewed Tinyland broker markdown, including broker-only slugs that are not checked into src/posts
• move Pulse AP demo fetches to hub.tinyland.dev and relabel checked-in post ingest as fallback/regression evidence only

Validation

• git diff --check
• npm ls --package-lock-only --depth=0 marked dompurify
• attempted targeted vitest, but this temp worktree dependency tree is incomplete and fails before tests on missing shiki from svelte.config.js; CI clean install is required

No local Playwright/browser smoke was run.