TypeScript 7's native port is incompatible with ts-node, so cdk.json was
switched to tsx — but it was invoked via `npx`, which fetches an
unpinned copy from the registry at synth/deploy time with no lockfile
entry or integrity check.
Declare tsx as a pinned devDependency (~4.23.0) and invoke the local bin
instead of npx. Update the README's cdk.json section to match. Verified
with `npm run build` (tsc 7.0.2) and `cdk synth` — both green.
