fix(ci): free-tests lane actually runs the make-pdf e2e gates

The 9 make-pdf/test/e2e gate tests probe make-pdf/dist/pdf,
browse/dist/browse, and the diagram-render bundle, then self-skip when
absent. The required free-tests lane never built any of them, so the
gates silently skipped on Linux for their entire life (verified: 9 of
14 skip, exit 0). make-pdf-gate.yml's justification for deleting its
Linux leg claimed the free lane covered this — it didn't.

- new build:gates script: exactly the three artifacts the gates probe
  (full bun run build compiles five binaries; ~60-90s tax on the only
  required check is not warranted)
- free-tests.yml: build:gates step + poppler-utils +
  fonts-noto-color-emoji (fonts must precede the first browse daemon
  launch — Chromium snapshots fontconfig at startup; verified live:
  a warm daemon renders tofu, a fresh one embeds NotoColorEmoji)
- make-pdf/test/e2e/ci-prereqs.test.ts: GSTACK_EXPECT_BINARIES=1 (set
  by the workflow) inverts the skip polarity in CI — dropping the
  build step or poppler fails the lane instead of re-opening the
  silent-skip hole

Pre-flight: all 9 gates green on Linux locally.

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

package.json

@@ -14,6 +14,7 @@
     "dev:make-pdf": "bun run make-pdf/src/cli.ts",
     "dev:design": "bun run design/src/cli.ts",
     "build:diagram-render": "cd lib/diagram-render && bun install && bun run scripts/build.ts",
+    "build:gates": "bun build --compile make-pdf/src/cli.ts --outfile make-pdf/dist/pdf && bun build --compile browse/src/cli.ts --outfile browse/dist/browse && bun run build:diagram-render",
     "gen:skill-docs": "bun run scripts/gen-skill-docs.ts",
     "gen:skill-docs:user": "bun run scripts/gen-skill-docs.ts --respect-detection",
     "dev": "bun run browse/src/cli.ts",