feat(i18n): pre-install en-US (built-in fallback)

Embed web/mirror/lang/en-US.pjson as kBuiltinEn and make the built-in fallback
locale-aware: when the active locale is en-* and no pack is on disk (or a key is
missing), PMM renders English instead of falling back to zh-CN. Verified:
LANG=en-US.UTF-8 + no pack -> help.usage = 'usage:'; zh-CN still 用法:.
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JGZYES
2026-09-06 00:04:09 +08:00
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