feat: close the browser boundary and add the generic UI host loader

Two gaps stood between the module UI contract and a host that could use
it. ADR 0023 recorded the first honestly but never diagnosed it: a module
UI file compiled with `"types": []` could still typecheck `process.env`.
The second was simply missing — nothing imported a compiled artifact,
checked its identity, mounted it, or tore it down.

The leak was one type-only import, bisected against the real type graph
rather than guessed. `@ekklesia/module-ui-sdk` imported
`type { EkklesiaPrincipal } from "@ekklesia/contracts"`. Importing
contracts/identity alone is clean and importing Zod alone is clean, but
contracts exposes a single barrel entry, so that one import reached
module-database, whose Drizzle types re-introduce Node's globals. The
runtime was never affected — the import was erased and host-context.js is
`export {}` — but a type-graph leak is still a real boundary failure,
because it silently permits code that cannot run in its target
environment.

The fix is not a narrower import path. The SDK now has no dependencies at
all, so there is no graph to leak through and no future contracts change
can reopen this. It owns `ModuleUiPrincipal`: the same shape ADR 0022
chose, with the Zod brand dropped. That brand encodes a server-side
parsing guarantee a browser cannot produce, and it is the same reason
`permissions.has` already took a plain string. An Ekklesia host still
passes its own `EkklesiaPrincipal` straight through — a branded string is
assignable to `string` — while a standalone host builds the literal.
Proven the way an author experiences it: two fixtures compiled under the
exact project a module uses, one supported and one deliberately reaching
for `process`, with the second required to fail on that specific name.

`@ekklesia/module-ui-host` is the generic loader, in its own package
because the SDK is bundled into every module artifact and host-only
loader code has no business shipping there. The boundary checker enforces
both directions: modules may not depend on the loader, and the loader may
depend only on the SDK plus dev-time tooling.

The distinction that shaped the design: Core resolves `entrypoints.ui` to
an absolute path on the server's disk, and a browser needs a URL. A
loader that accepted the path would test green on a server and fail in
every real host, and would quietly own asset publication. So the loader
takes a browser-loadable URL and knows nothing about how one is obtained.
Ekklesia has no route that publishes installed artifacts, so this is
precisely: loader complete, publication deferred. Nothing can mount an
installed module UI yet, and the docs say so rather than implying
otherwise.

Dynamic import is `(url) => import(url)` with a non-analyzable specifier,
kept as a runtime import so a host does not statically bundle every
installed module or need a hardcoded registry. It is injectable for
bundlers needing their own escape hatch, and deliberately not `eval` or
`new Function`, which would cost a host `unsafe-eval` in its CSP.

Identity is validated through the SDK's existing parser before `mount`
runs, so a stale or mismatched bundle never renders. Contract violations
propagate unwrapped; the loader adds only import and mount failure codes,
sanitized, with the cause attached. Unmount is idempotent, safe when a
module returned no cleanup, and marks an instance unmounted before
running cleanup so a throwing cleanup surfaces once and never re-runs. A
failed mount registers no cleanup. Each instance owns its own state, so
two mounts coexist. The loader never mutates the host's container — not
on success, failure, or unmount.

Tests prove all of it against real module URLs (`data:`, which both
browsers and Node import natively) rather than filesystem paths,
including one artifact built by the canonical Pass 2 pipeline and run
under both an Ekklesia-shaped and a standalone-shaped context. Core,
apps/api, and the contracts package are untouched; discovery remains
inert.

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

README.md

@@ -287,9 +287,24 @@ from source, validate it with `parseModuleUiExport`, mount it into a real DOM, a
 output, and unmount it — so a UI that cannot be produced fails CI, and no generated file is ever
 committed. See [ADR 0023](docs/architecture/decisions/0023-module-ui-build-pipeline.md).
 
-**Still no host.** A module UI can be authored, built, and tested; **nothing serves one to a user
-yet.** The Dashboard shell, navigation, a design system, a stylesheet loading contract, and the
-standalone host do not exist.
+Phase 6B pass 3 closes the last two foundational gaps. **The browser boundary now actually holds**:
+`@ekklesia/module-ui-sdk` had one type-only import of `@ekklesia/contracts`, whose barrel pulled
+Drizzle's types — and with them Node's globals — into every module UI program, so `process.env`
+typechecked inside browser-only code. The SDK now has **no dependencies at all**, and owns a plain
+browser `ModuleUiPrincipal` instead of Core's Zod-branded server type; a compile test proves that
+supported UI source compiles and that `process` does not. **And there is now a generic host loader**:
+`@ekklesia/module-ui-host` imports a published artifact from a browser URL, validates its id and
+version against the manifest *before* mounting, mounts it into a host-owned container, and returns an
+idempotent `unmount()`. A module may never depend on it — it is host infrastructure, and the boundary
+checker enforces that. See
+[ADR 0024](docs/architecture/decisions/0024-browser-safe-ui-sdk-and-host-loader.md).
+
+**Still no host, and one specific reason why.** Core resolves `entrypoints.ui` to a path on the
+*server's filesystem*; a browser cannot import that. **Ekklesia has no route that serves an installed
+module's artifact at a browser URL**, so nothing can mount an installed module UI yet — the loader is
+complete, asset publication is not, and that is the next pass's first job. The Dashboard shell,
+navigation, a design system, a stylesheet loading contract, and the standalone host also do not
+exist.
 
 **Security model.** Installed modules are trusted compiled backend code that runs arbitrary
 JavaScript in the API process and shares its PostgreSQL role — table prefixes, manifests, permission