Slim borg: one small app that checks the local twins against a real deploy #855

The local twins copy Cloudflare's shapes. Nothing checks that they copy its behaviour, and the gaps only show up after a deploy. Slim borg is the thin rig for that: one small Worker that uses every binding once, and one suite that runs the same checks against any URL.

In the first PR (apps/borg):

• Checks: kv, d1, d1-batch (atomic), r2, do (input gate, one object per id), ws (hibernation attachment), queue (consumer delivery)
• Tier 1, free, every PR: bun test test builds the real dev host and runs the suite on the local twins
• bun borg.ts <url> runs the same suite against a deploy or a running host
• The two twin bugs it found on its first run, fixed: D1 batch() wasn't atomic, and local Durable Objects had no input gate

Still to do:

• Tier 2, free tier, nightly or on a label: deploy to workers.dev, run borg.ts, destroy, publish the grid. This needs a Cloudflare token as a CI secret, which only the owner can add.
• Check that the whole stack (Queues and R2 included) deploys on a Free-plan account, or put the paid-only bindings behind a flag
• A /limits check: go over CPU, memory and bound parameters on purpose, and report where the twin is looser than the edge
• The box target (build box) in tier 1
• Guard rails for any run that costs money: a cost cap from plan, destroy even when checks fail, and a sweeper for anything tagged borg older than an hour