feat: give module UI real stylesheets, and every artifact its own root

ADR 0023 refused stylesheet imports for a good reason: esbuild would emit a
`.css` sidecar beside the artifact, nothing would declare it, no host would
fetch it, and it would work in a local test and silently do nothing in a real
host. Inline style objects were the whole story. They cannot express
`:hover`, `:focus-visible`, `:disabled`, `@media`, or `@keyframes` — so an
accessible focus treatment was not expressible, and the shared elements added
in the previous commit are built from real CSS for exactly that reason.

Compile the stylesheet into the artifact instead of beside it. A module
imports plain `.css`; the canonical builder resolves local `@import`s through
a nested bundle and turns each file into its compiled text, which the author
declares as `defineModuleUi({ styles })`. One declared file still contains
everything: nothing extra to publish, nothing to fetch, and no way for CSS to
become an asset nobody serves. The build additionally fails if a CSS output
ever appears beside an artifact, so a future loader change cannot bring the
sidecar back. Two rules are enforced with their own codes: a module stylesheet
may reference `--ekklesia-*` and never declare one, and every `@import` must
resolve locally or the artifact would fetch styles at run time. Plain `.css`
only; other dialects are a toolchain decision needing their own ADR.

Then make the isolation real. The generic loader mounts every browser artifact
— primary, display, and later a contributed renderer — inside an open Shadow
DOM root it creates, installs the artifact's stylesheet there, and hands the
artifact an element inside it. Shadow DOM is the one mechanism a browser
actually enforces; a class prefix or a "scope your selectors" instruction is a
convention one mistake defeats, and the failure mode is a module silently
restyling the Dashboard. It applies unconditionally, including to artifacts
with no stylesheet, because a conditional boundary is one a module can fall
out of by editing a source file. Both wrappers are `display: contents` through
base rules installed before the artifact's own, so layout is unchanged and a
full-viewport display can still override `:host`. Unmount removes the whole
root in a `finally`, so a cleanup that throws cannot leave styles behind, and
remounting cannot accumulate a second copy.

This is a correctness boundary for styles and is not described as a sandbox:
the root is `open` precisely because module code is trusted, same-realm code
that could reach `document` anyway.

An artifact's DOM is no longer a direct child of the host's element, so the
mount ABI is versioned explicitly. `defineModuleUi` stamps
`MODULE_UI_ABI_VERSION` so it can never drift from the SDK the artifact
compiled against, and a version-1 artifact fails with its own code — "rebuild
your module", not a typo hunt. No dual acceptance, matching every manifest
cutover here. Migration is one `pnpm build`.

`pnpm boundaries` allows `.css` only under `src/ui/` and `src/display/`: a
stylesheet anywhere else in a module is the undeclared sidecar again, just
relocated.

Co-Authored-By: Claude Opus 5 <[email protected]>

docs/architecture/repository-boundaries.md

@@ -20,7 +20,11 @@ These rules map platform ownership to filesystem and package boundaries. The exe
 - A module may own UI ([ADR 0021](decisions/0021-modules-as-hostable-products.md)) and authors it in
   React and TypeScript ([ADR 0022](decisions/0022-module-ui-authoring-and-host-contract.md)). A
   module may therefore contain `.tsx` source and depend on `react`, `react-dom`, and
-  `@ekklesia/module-ui-sdk`. It still cannot contain `.jsx`, stylesheets, single-file components,
+  `@ekklesia/module-ui-sdk`. Since
+  [ADR 0040](decisions/0040-module-stylesheets-and-artifact-isolation.md) it may also contain plain
+  `.css`, but **only** inside a browser-artifact source root the canonical builder compiles
+  (`src/ui/`, `src/display/`); a stylesheet anywhere else in a module is a file no build reads and no
+  host loads. It still cannot contain `.jsx`, other stylesheet dialects, single-file components,
   Astro/Marko/MDX/HTML views, or SVG presentation assets, and cannot depend on any other UI framework
   (including `react-router-dom`, `react-native`, and `@vitejs/plugin-react`) or on any shared role
   beyond the two approved design packages named below. A module needs no bundler plugin: `@ekklesia/module-ui-build` — a `tooling` package,
@@ -51,6 +55,14 @@ These rules map platform ownership to filesystem and package boundaries. The exe
   Node's global declarations — into browser code. Core still owns the containing
   application shell and all platform policy, and module UI reaches the host only through
   `@ekklesia/module-ui-sdk`.
+- A module stylesheet is compiled **into** the one declared JavaScript artifact as text and installed
+  by the host at mount; there is no sidecar file, no second publication route, and no manifest field
+  for one. Every browser artifact is mounted inside its own Shadow DOM root the generic loader
+  creates, so one module's styles cannot reach the Dashboard, another module, or another renderer,
+  and the host's cannot reach in. That root is a correctness boundary for styles, never a sandbox:
+  module code remains trusted, same-realm code. The browser mount ABI is versioned
+  (`MODULE_UI_ABI_VERSION`) and a mismatched artifact fails closed rather than mounting under guessed
+  semantics.
 - Manifest v6 permits independently optional primary (`entrypoints.ui`) and full-viewport display
   (`entrypoints.display`) artifacts. Modules own both feature experiences; surfaces own their
   containers, routes, navigation/chrome, credentials, error states, and publication wiring. The