The experimental subpath pointed at `src/mosaic/styles`, which is the StyleX
build barrel: it pulls every migrated component into the graph so one stylesheet
can be extracted. Publishing it made all of those components, and the headless
primitive types behind them, part of the API. Give the export its own narrow
entry instead.

`@clerk/headless` is private, so leaving it external published a dependency that
404s on install. Bundling it broke differently: the declaration builder cannot
follow the re-export chains in its emitted `dist/*.d.ts`. Correct the
`tsconfig.mosaic.json` paths so it resolves to source (primitives live under
`src/primitives`, so the old wildcard missed menu, popover, dialog and tabs and
silently fell back to `dist`).

StyleX moves to devDependencies and gets bundled: it compiles away at build
time, so consumers should not inherit our version.

Keeps the experimental surface to the component and the props it takes;
everything behind it stays internal.

Locks the props a consumer writes against `@clerk/nextjs/experimental/mosaic`: what the switcher modes are called, that a routing URL cannot ask for a modal, and that a menu row or profile page has to be one of its two shapes.

Floating UI arrives through the bundled @clerk/headless primitives, so leaving it external made it the one install this entry still demanded. SDKs inline this bundle so consumers need nothing beyond React and @clerk/shared.

…r trees

Left external, the re-export resolved from the consumer's tree at runtime, which made @clerk/ui a dependency and installed its whole graph (Emotion, the Solana wallet adapters, ...) for every consumer, Mosaic or not.

The two tests guarding `mode` and `modePriority` against each other's
wording go away with the divergence they guarded.

Pins the return shape: `{ name, slug? }`, `null` to decline, no arguments in.

…ests