test(make-pdf): emoji render gate (pdffonts + pixel proof)

pdftotext is a false oracle for emoji: Skia preserves the Unicode in the
text cluster even when the glyph drew as .notdef tofu, so extraction passes
on a broken render. The gate instead asserts (1) pdffonts shows an emoji
family embedded and (2) pdftoppm rasterizes the page to color (measured
~1650 saturated pixels vs ~0 for tofu). pdfimages is not used: macOS embeds
color emoji as Type 3 fonts, so it lists nothing even on a correct render.
Adds resolvePopplerTool() (DRY resolver, returns null for clean skips) and
a fixture exercising FE0F variation-selector emoji. Skips cleanly when
poppler tools or a color-emoji font are unavailable.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

make-pdf/test/fixtures/emoji-gate.md

@@ -0,0 +1,12 @@
+# Emoji rendering gate 😀
+
+This fixture exists to prove that emoji code points render as real color
+glyphs in the output PDF, not as `.notdef` tofu boxes (▯).
+
+Color emoji on one line: 😀 ❤️ 🚀 ✅ 💡
+
+A variation-selector sequence (FE0F) renders color: ❤️ — the bare code point
+❤ is text-style. Both must come from a font in the cascade, never tofu.
+
+Non-emoji Unicode (unchanged, regression guard): em dash —, times ×, arrow →,
+bullet •, ellipsis …