`pluralize` hard-codes `en-US` and renderers are inconsistent

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Problems

Spec vs reference renderer — The specification does not define which locale drives plural categories, while the reference implementation hard-codes en-US. Renderer SDK authors have no normative target and cannot tell integrators what behavior is “correct.”

1. WebCore hard-codes en-US
basic_functions.ts line 326: new Intl.PluralRules('en-US').select(args.value) — wrong category for any non-English content (e.g. Polish select(2) → few, not other).

2. Flutter ignores CLDR entirely
basic_functions.dart uses if (count == 0) / if (count == 1) / other — no Intl.PluralRules equivalent, always wrong for languages like French (0 → one) or Arabic.

3. No locale in the spec
pluralize.args has no locale field and uses unevaluatedProperties: false, so agents cannot pass a locale even if they want to.

Fix

• Add optional locale (BCP 47) to pluralize.args in the spec.
• Update WebCore to use args.locale when present, fall back to client locale, then en-US.
• Fix Flutter to read value (not count) and use a proper CLDR plural lookup.

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Related: flutter/genui non-conformance

The Flutter/genui renderer has two additional issues beyond what's described above, tracked separately:

• pluralize reads wrong key: args["count"] should be args["value"] flutter/genui#902 — pluralize reads args["count"] instead of the spec-required args["value"], making the function completely non-functional regardless of input.
• pluralize schema and implementation missing two, few, many branches (v0_9 spec non-conformance) flutter/genui#903 — pluralize schema and implementation are missing two, few, many branches entirely. Unlike WebCore which hard-codes en-US but still reaches all six CLDR categories, genui has no code path for them at all.

Both genui issues need to be resolved alongside the fixes proposed in this issue.