Next.js: Raise the supported floor to 15

[obvious-autobuild]

Why

Storybook 11 is raising the supported framework floors. For Next.js the supported floor moves from 14.1 to 15: the Next 14 cohort can stay on Storybook 10 or follow the migration guide, and the compatibility code that existed only for Next 14 is real dead weight in @storybook/nextjs — a package that is itself deprecated in favor of @storybook/nextjs-vite (removal announced for Storybook 12).

What

One floor, four layers, one commit per layer:

• Peers — next peer range raised to ^15.0.0 || ^16.0.0 in @storybook/nextjs, @storybook/nextjs-vite, and @storybook/vite-plugin-storybook-nextjs.
• Upgrade-time block — the next entry in the autoblock map moves 14.1.0 → 15.0.0. The blocker message links the new MIGRATION.md anchor (#nextjs-require-v15-and-up) instead of the stale Next.js 13 upgrade guide. --force still bypasses, per the existing autoblock contract. A new unit test covers the contract: next 14 → blocked with message + anchor; next 15/16 → pass.
• Dead code — with the floor at 15:
  • src/compatibility/ (compatibility-map + draft-mode.compat) is deleted. Its only entry was the '<15.0.0' alias table, and the ./compatibility/draft-mode.compat export goes with it (package.json exports + build config). The compat-alias spread in export-mocks and the compat-path warning filter in aliases/webpack.ts — both added by "Nextjs: Add support for Next 15" for the Next 14 shim — are removed too.
  • The always-true isNext14orNewer term in preset.ts is dropped: the supported floor is 15. Compiler selection is otherwise unchanged from next for every supported version — useSWC = forceSwcTransforms || !hasBabelConfig — so the Babel compiler path, the babel preset property, and the whole src/babel/ tree stay intact (a project shipping .babelrc/babel.config.js without forcing SWC transforms still compiles through Babel).
  • The always-true >=13.0.0 legacy-image check collapses into an unconditional alias assignment.
• Docs — "Next.js: Require v15 and up" subsection added to the existing "From version 10.x to 11.0.0" section in MIGRATION.md (alongside the Vitest 4 and Vite floor entries), with a matching table-of-contents entry — landing in the same PR as the code that links the anchor.

How to Review

• The behavioral core is preset.ts: webpackFinal keeps next's compiler selection for all supported versions — only the always-true isNext14orNewer term was dropped. The Babel compiler path and babel preset property are restored unchanged after review feedback. The only remaining version check in the framework is the pre-existing isNext16orNewer check in previewAnnotations — a Next 16 concern, out of scope.
• font/babel/ (the TransformFontImports babel plugin) and the babel-loader dependency are consumed again by the restored Babel path — unchanged from next. The vendored <15.0.0 copy under vite-plugin-storybook-nextjs's next-mocks stays untouched (self-contained, separate surface).
• react / react-dom / @types/react peer ranges are intentionally untouched — that sweep belongs to the React floor PR.
• No sandbox template changes: the nextjs 15/prerelease templates stay per the spec (D8).

Test Evidence

• yarn test autoblock — 4 files, 31 tests pass, including the new block-dependencies-versions.test.ts (next 14 → blocked with message + anchor; 15/16 → pass).
• yarn test code/frameworks/nextjs — 2 files, 14 tests pass after the deletions.
• scripts/build-package nextjs — build succeeds; fresh dist/ bundles the Babel compiler path again and has zero references to draft-mode.compat or the deleted compatibility/ tree.
• Per-package typecheck (scripts/check/check-package.ts) — clean for code/frameworks/nextjs and code/lib/cli-storybook.
• oxlint clean on all changed files.

Manual testing

Automated coverage carries this change; no manual steps are strictly required:

• yarn test autoblock — new unit test covers the Next.js 15 blocker (next 14 → blocked with the migration link; next 15/16 → pass; --force bypasses).
• yarn test code/frameworks/nextjs — framework unit tests pass after the compatibility deletions.
• scripts/build-package nextjs — package build succeeds with the restored Babel path bundled and zero references to the deleted draft-mode.compat tree.
• Optional smoke test: generate a sandbox against a Next.js 15 app and verify stories boot through the config-driven compiler selection — SWC by default, and Babel for a project that ships a .babelrc.

QA

Visual evidence not applicable — no rendered UI, visual state, or interactive workflow changes.

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