fix: add publish integrity check for @aws-blocks/blocks docs sync

[svidgen]

Problem

The @aws-blocks/blocks package includes synced docs from sibling building blocks (via sync-block-docs.mjs prebuild). When those sibling docs change, the blocks tarball content changes — but changesets doesn't know to bump blocks itself. This causes the Publish Packages workflow to fail with:

@aws-blocks/blocks@0.2.0 already exists with different content

Fix

1. Changeset — adds a patch bump for @aws-blocks/blocks to unblock the current publish failure
2. CI check — a new workflow job that catches this at PR time: packs the blocks package, compares its integrity hash against npm, and fails with a clear error if content diverges without a changeset

How the check works

• Runs sync-block-docs.mjs (prebuild)
• npm pack the blocks package
• Compares SHA-512 integrity against what's currently published on npm
• If they differ and no changeset includes @aws-blocks/blocks → fails with instructions