- new Intl.NumberFormat("en-US").format(x), the callable spelling, and x.toLocaleString("en-US") lower to one runtime formatter with default options: decimal notation, 0-3 fraction digits rounded half-up on the SHORTEST round-tripping decimal (ICU's rounding input, probed against Node — format(1.0005) is "1.001" though toFixed(3) answers "1.000"; 1e23 prints the shortest form's zeros, not the exact expansion), "," grouping, "∞"/"NaN" texts, and "-0" for negatives rounding to zero. The en-US/latn symbols are the whole embedded locale surface — no ICU is linked.
- The Ryū digit core factors out of scr_f64_to_str as scr_f64_digits (same bytes; the 1M-double oracle gate stays green) and feeds the formatter; a 30,000-double ad-hoc fuzz across magnitude regimes matched Node byte-for-byte on both backends.
- Every unlowered Intl form fences by NAME with its own reason: the environment-default locale a compiled binary cannot carry (no-locale forms), other locales and the rest of the namespace (DurationFormat, DateTimeFormat, PluralRules, getCanonicalLocales — ICU data the binary does not embed; Node 24's DurationFormat additionally throws V8-internal Temporal errors with embedded source paths, so byte parity there is a version-dependent landmine), options bags, formatter values (the composed form is the lowered shape), and non-number format arguments.
- Differential fixture 2562 pins the rounding discriminators, grouping, ±0/NaN/∞, tiny and huge edges, and both spellings byte-identically vs Node on both backends; the new intl diagnostics fixture pins every named fence, and the Intl hints across the identifier/member/type chokepoints now teach what lowers.
