test(smoke): boot a scaffolded board at the depth Vercel deploys it

[jouwdan]

Every board bug found this week shipped through forum-web build --at-root, and no CI job ever built or booted a board that way.

• MEI-130 — the theme catalog was never registered, so boards rendered default.latestThreads.heading
• MEI-131 — the Tailwind scan roots resolved to nothing, so boards shipped 17KB of preflight and no component rules
• MEI-133 — /install could not finish

All three were found by a person looking at a deployed page. The checks added for the first two run at .meith/app only, and rebaseGlobalsCssSources computes a different path at each depth — ../../node_modules/@meith at depth zero versus ../../../../node_modules/@meith at depth two — so depth two was never evidence about depth zero.

What changed

The boot and its assertions are one helper now, run against two boards:

.meith/app	the way a self-hoster gets it — unchanged
project root, --at-root	the way Vercel deploys it — new

Both are held to the same three things: it renders <main>, no message key reaches the page as text, and the stylesheet carries rules for classes only @meith/ui and @meith/theme-default produce.

Why a second board rather than rebuilding the first

Rebuilding would save the pack and one npm install. It would also test two materialization depths coexisting in one workspace, which no real board does — so a failure there would be the smoke failing something no user can reach. Given how much of this week went on gates that were wrong rather than missing, the extra minute is worth it.

What a green run does not prove

Both written into docs/development.md rather than left to be assumed later:

• The at-root board boots through the standalone server, because output: 'standalone' is skipped only when VERCEL is set and CI is not Vercel. Vercel packages its own functions. This proves materialization, Tailwind and the catalog at depth zero — not Vercel's serving shape.
• It is a fresh board, so nothing here says the two depths can coexist.

Verification

Run locally end to end, both boards green:

== the board materialized at .meith/app rendered / ==
== every class the board rendered has a rule ==
== a second board, materialized the way Vercel deploys it ==
== forum-web start at the project root ==
== the board materialized at the project root rendered / ==
== every class the board rendered has a rule ==
✓ board-workspace-smoke: a scaffolded, externally-installed board builds and boots

This box has no Docker, so Postgres was PGlite over the wire protocol. The first attempt failed with unnamed prepared statement does not exist on the existing depth-two boot — that was the substitute database accepting 30 connections; the e2e harness serves the board with maxConnections: 1 for the same reason. Nothing in the product: packages/db/src/client.ts already sets prepare: false on every connection. CI uses a real postgres:18-alpine and needs none of this.

docs:links:check passes. The CI job comment and step name were updated to say it now boots at both depths.

Fixes MEI-132.

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