the built SPA had no tailwind, and the build could destroy itself

three fixes to last night's build switch, all found by using it.

THE CSS. bunfig.toml declares the tailwind plugin under [serve.static], which
applies to bun's static SERVING — the html-import path the app used until
yesterday — and not to a programmatic Bun.build(). so the first build emitted
the xterm css and no tailwind at all: layout intact, every utility class
missing. a plugin list is not inherited from bunfig; it has to be passed. css
goes 110KB to 278KB, 1127 --tw- variables, .flex present, --color-duck present.

THE BUILD DIRECTORY. clearing it before building was meant to stop 20MB of
content-hashed chunks accumulating per install, and instead meant a FAILED build
left nothing — the exact opposite of the promise in the comment directly above
it. it was also a race: two builds overlapping had one process's rm delete the
other's shell, leaving js and css with no html and a 503 that read as a build
failure when the build had succeeded.

now it builds into build.next/ and swaps only a complete, successful build into
place, and refuses to swap one that produced no shell at all — a build can
report success and emit no html, and serving that is worse than serving the
previous one.

AND IT SAYS WHICH PATH IS SERVING. "is it serving the build I just made, or the
one bundled at import?" was answerable only by hiding the shell and watching for
a 503, which is how it got answered once. the distinction matters precisely
where it is hardest to see: the html import is fixed when the module graph
loads, so an install would rebuild build/ and serve something else entirely.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/server.tsx

@@ -225,6 +225,18 @@ const serveBuilt = async (req: Request): Promise<Response> => {
 const isProduction = process.env.NODE_ENV === 'production';
 const spaRoutes = isProduction ? { '/': serveBuilt, '/*': serveBuilt } : { '/': officerWeb, '/*': officerWeb };
 
+// Said out loud, because "is it serving the build I just made, or the one bundled at import?" is otherwise
+// answerable only by hiding the shell and watching for a 503 — which is how it was answered once.
+//
+// It matters because the two behave differently in exactly the case that matters: the HTML import is
+// bundled when the module graph loads and can never change, so installing a plugin would rebuild `build/`
+// and serve something else entirely.
+console.log(
+  isProduction
+    ? `[web] serving the built SPA from ${BUILD_DIR} — a plugin install rebuilds it`
+    : '[web] serving via the HTML import (dev) — HMR is live, and `build/` is not used',
+);
+
 async function upgradeWs(
   req: Request,
   server: any,