Intl gaps: RelativeTimeFormat numeric:"auto" word forms, and instanceof through an Intl subclass #6960

Two pre-existing Intl divergences, both reproduced on released perry 0.5.1220 (so not from any in-flight size work) — found while probe-testing #6959.

1. RelativeTimeFormat with numeric: "auto" doesn't use word forms

new Intl.RelativeTimeFormat("en", { numeric: "auto" }).format(-1, "day")
// node:  yesterday
// perry: 1 day ago

numeric: "auto" should substitute the CLDR relative word forms (yesterday/today/tomorrow, last/this/next week…) instead of always rendering the numeric form.

2. instanceof fails through an Intl subclass

class MyNF extends Intl.NumberFormat {}
new MyNF("en-US") instanceof Intl.NumberFormat
// node: true — perry: false

The subclass instance formats correctly (new MyNF("en-US").format(99) → 99), so construction and the brand both work; only the instanceof brand probe misses the subclass case. Same family as #6924 (native-base subclass gaps).