Say why source mode wants an exports list

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One error message. No behaviour change.

emitSourceManifest threw must declare its public Layout exports, which reads as a missing required field and sends you off to write the list. The question worth asking first is whether you wanted mode: "source" at all, because convention-based discovery needs no such list and docs/getting-started.md says exactly that: "No authored component manifest is required."

It also said nothing about what keeping the list costs. Nothing compares it to the barrel, so a rename that misses it leaves the manifest naming components that no longer exist while rejecting the ones that do, silently and with no diagnostic anywhere.

That is how this was found. A library on source mode renamed eleven components across its source, its recipes and its barrel, rebuilt from a wiped dist, and the manifest still said Callout. Everything typechecked, the build was green, and the application compiler would have rejected <Alert> while accepting a name that no longer existed. The same list had also never heard of two components added that morning.

The new message names the trade and points at both exits:

layouts.library.json: source mode needs an "exports" array naming this package's public components.

Only `mode: "source"` needs one. Convention-based discovery derives the manifest from
every `*.layout.tsx` it finds, and emits richer entries the application compiler can
verify; source mode generates alongside a package whose public surface it cannot see,
so the list is how it learns the names.

If this package does not need adjacent generation, delete the config and let discovery
do it. If it does, the list has to be kept in step with the barrel by hand: nothing
compares them, and a rename that misses it leaves the manifest naming components that
no longer exist while rejecting the ones that do.

Suites unchanged and passing: library 7, application 14.

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#5 is stacked on this branch. Merging this one first keeps that diff clean.

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Folded into #5, which contains this commit plus the Solid 2 work. One PR instead of two.