wave E stage 3: demos run in the app's own Chromium (browser-service helper)

Demos execute JS and need a browser; the app's own binary now provides
one: main.cjs boots a hidden window + CDP port under --browser-service
(before the single-instance lock - a helper is deliberately a second
instance of this binary), lifetime tied to the server's stdin (EOF =
quit, no orphans). headless.ts connects over CDP as PRIMARY (owner
ruling 2026-10-10), system Chrome stays the fallback, graceful refusal
when neither. Electron's CDP has no Target.createTarget, so the helper's
pre-created window is THE page, reused and reset per job behind a
serialized queue. PDFs and SVGs never reach a browser anymore (native
since stages 1-2); stale Chrome wording in model-facing tool copy made
engine-neutral. Live-proofed end-to-end from the dev tree (helper seen
by process watch, js-ran 42 probe, no orphan). Gates 1523/1523.

src/agent/subagents/pdf-reader.ts

@@ -167,7 +167,7 @@ export async function runPdfReaderAgent(
 	const maxIterations = opts.maxIterations ?? MAX_ITERATIONS;
 	const pdfPath = input.path;
 
-	// Deterministic upfront failure: unreadable / unparseable / no Chrome
+	// Deterministic upfront failure: unreadable / unparseable
 	// for renders — the brief never starts on a broken source.
 	const openError = await probeReadable(pdfPath);
 	if (openError) {