docs: add MIGRATION_v5_to_v6 stub + trigger 6.0.0-beta.1 release-please bump (refs #802)

MIGRATION_v5_to_v6.md

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+# Migration v5 → v6
+
+> Beta line. Surfaces here MAY change between beta increments. Track this
+> file as features land; sections will be added per-release rather than
+> upfront.
+
+## Why a major version
+
+v6 carries the type-surface changes that come with AdCP 3.1 — most
+notably the canonical-formats projection layer (#741), which alters
+the `ProductFormatDeclaration` discriminator and adds the
+`adcp.canonical_formats` public module. Adopters pinning `adcp~=5.7`
+will keep getting the v5 surface; opt into the beta with
+`pip install --pre adcp` or `pip install adcp==6.0.0b1`.
+
+## Installing the beta
+
+```bash
+pip install --pre adcp
+# or pin explicitly
+pip install adcp==6.0.0b1
+```
+
+PyPI's default resolver excludes prereleases unless `--pre` is set, so
+existing `pip install adcp` calls in production continue resolving to
+5.7.x.
+
+## What's NOT changing
+
+- `ADCP_VERSION` is the upstream protocol version (currently
+  `3.1.0-beta.3`) and is independent of the SDK package version. v6 of
+  the SDK can run against multiple `ADCP_VERSION` values; the schema
+  cache and generated types are pinned via that file, not via the
+  package version.
+- The CI schema-sync drift gate remains gated on stable upstream
+  versions (`.github/workflows/ci.yml`).
+
+## Per-release entries
+
+(Add sections here as breaking changes land. Format: heading per
+released beta increment, bullet list of breaking surfaces with
+migration recipe.)