DEV-2100: namespace docs-example buckets

[demtario]

Summary

Task: https://app.clickup.com/t/9015210959/DEV-2100

• require a docs branch and write each import to its version-specific bucket
• resolve develop from npm dist-tags.next and make registry/version failures fatal
• add bucket-resolution and fixture tests, including sibling-bucket preservation

Test plan

• pnpm test
• pnpm typecheck in packages/runtime
• pnpm typecheck in apps/authoring

Deferred

Generated docs-example artifacts remain unchanged. Regenerate and remove the legacy flat snapshot only after the bucket-aware client changes land in the feature branch.

PR intentionally merged into another feature branch, to not break production logic with partial changes.

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Note

Medium Risk
Changes the docs import contract and on-disk layout (client bucket wiring is deferred), and develop imports now depend on live npm registry data with strict failure modes.

Overview
Docs-example imports are version-bucketed instead of a single flat docs-examples/ tree. import-docs.mjs now requires --docs-branch: prod-docs/<major.minor> writes under <bucket>/ (e.g. 18.0) and develop writes the next bucket. Regenerating one bucket replaces only that directory so sibling snapshots stay intact.

Handsontable version baking is centralized in docs-import-config.mjs: release branches use the sibling handsontable/package.json semver; develop uses npm dist-tags.next, with invalid or missing registry data treated as fatal import errors. Manifests and example entries record bucket, docs branch, and hotVersion.

The authoring app gets resolveDocsBucket in @handsontable/demo-runtime to map a selected HOT version to a bucket (exact match for next, major.minor for releases, no silent fallback to next). Docs and pnpm test (runtime build before pipeline tests) are updated; pipeline and integration tests cover branch rules, version resolution, bucket preservation, and bucket resolution.

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