feat(cms): provide compat ctx as 3rd arg to section loaders (#305)

[JonasJesus42]

Fixes #305. deco.cx (Fresh) section loaders use a 3-arg (props, req, ctx) signature, but the framework invoked loaders with only (props, req), so migrated loaders read undefined and threw — rendering sections blank on every migrated site. Rather than delete ctx (which would force rewriting every loader by hand), the framework now assembles a real compat ctx (device from the request UA, server-side ctx.invoke.* via self-fetch, per-app state via getAppState, and ctx.response.headers) and passes it as the 3rd arg through withPageContext — covering all four loader paths — with compose/withSectionLoader forwarding it and a parametrized createAppInvokeWith. On the migration side, a new ctx-compat transform optional-chains ctx.* reads so unconfigured app state degrades to undefined instead of crashing, the { seo: {} } as any SEOPDP stub (a D3 violation) is replaced with the real ctx, and the migration skills docs are updated. Validated by blocks (537) + transform (8) unit tests, a clean typecheck, and an end-to-end run on portal-davinci where the page renders with no ctx-undefined errors.

🤖 Generated with Claude Code

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Summary by cubic

Provide a real compat ctx as the 3rd argument to section loaders and add a codemod to optional‑chain ctx.* reads. This addresses (#305) by matching deco.cx (Fresh) loader APIs so migrated sections stop rendering blank.

• New Features

  • Added buildSectionLoaderContext(req) to supply ctx.device (from UA), ctx.invoke.* (server-side self-fetch), per-app state (ctx.vtex, ctx.salesforce, …), and ctx.response.headers.
  • Threaded the compat ctx through all loader paths via withPageContext; compose and withSectionLoader now forward it.
  • Introduced createAppInvokeWith({ basePath, fetcher }) and routed ctx.invoke through it with RequestContext.fetch.
  • Exported SectionLoaderContext and builder from @decocms/blocks/cms; updated SectionLoaderFn and FnContext types.
  • Added unit tests for context building, invocation, and loader plumbing.

• Migration

  • New ctx-compat transform: optional-chains all ctx.* reads in files exporting a loader so missing app state degrades to undefined instead of throwing; added to the transform pipeline.
  • Loader templates now forward the real 3rd-arg ctx (removed the { seo: {} } as any stub) and pass it to IsEvents.
  • Docs updated to clarify that the 3rd-arg ctx is real and that ctx.response.headers is supported (writes are inert on dev/SPA serverFn).

^{Written for commit f6e9cd7. Summary will update on new commits.}