Migration guides for major versions

A breaking change without a migration guide forces every existing user to diff the changelog, read source code, and guess at the new equivalent. A CHANGELOG entry that says 'BREAKING: removed fetchData()' with no explanation of what replaced it is effectively a notice to abandon the project. For libraries and SDKs, major version bumps without migration guides directly translate to production outages when users upgrade: the old code fails, the new code is unknown, and the documentation provides no bridge. iso-25010:2011 analysability requires that version transitions be documentable without source inspection.

Create a migration guide for each major version in docs/migration/ or UPGRADING.md, with before/after code for every breaking change:

# Migrating from v2 to v3

## Breaking Changes

### `fetchData()` renamed to `query()`

**Before (v2):**
```ts
const data = await client.fetchData('users')

After (v3):

const data = await client.query('users', { format: 'json' })

The format parameter is now required.

Every breaking change must have its own before/after section. Link the migration guide from both CHANGELOG.md and the docs navigation. A changelog that names a removed API without showing the replacement fails this check.