9 tasks in 5 batches: D1 adapter, Worker entry, Container entry,
build command, integration and docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Worker + Container architecture with D1, Service Binding RPC
for communication, and hatk build --target cloudflare output.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Instead of emitting one log per invalid record during backfill, collect
counts by collection and emit a single validation_skips event per repo.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Assets 404 without /hatk/ base when served from hatk-dev.github.io/hatk/.
Remove this once hatk.dev custom domain is active.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>