F2: Default `length_ratio` band is wrong for Japanese and Arabic

[ChauCM]

Surfaced by a pilot test (Claude Code Opus 4.7 against examples/after/, 2026-05-24). Full feedback: dialect-feed.md.

Problem

Default length_ratio is [0.3, 2.5] for every locale without an override. In practice:

• Japanese routinely hits 0.15–0.25× because labels collapse: "Settings" (8 chars) → "設定" (2 chars). Three label keys in the pilot's Settings hit length_ratio warnings on first check, all legitimate.
• Arabic plurals routinely hit 2.6–3.0× because six CLDR categories pack into one string.

Every Flutter app shipping ja/ar will hit these warnings on first translate. The reporter added overrides in dialect.yaml:

length_ratio:
  ja: [0.1, 2.5]
  ar: [0.3, 3.0]

Fix

Ship these (or similar) as defaults so adopters don't see noise warnings out of the box. Document the per-locale rationale near the defaults so the bands aren't mysterious.

Acceptance

• Default length_ratio table includes locale-specific bands for at least ja and ar (and any other locale where the global default is systematically wrong).
• The init-time canonical example's dialect.yaml no longer needs to override these.

[ChauCM]

A second pilot (Claude Sonnet 4.6, run 2026-05-24, archived feedback in examples/after/dialect-feedback.md) hit the same default-bound surprise for Chinese:

"Notifications" → "通知", "Language" → "语言", "Settings" → "设置" all tripped the default [0.3, 2.5] ratio. These are correct translations — Chinese characters carry far more meaning per glyph than English letters.

So the CJK scope here should include zh and ko alongside the originally-reported ja/ar. Suggested defaults (from the second pilot):

length_ratio:
  zh: [0.1, 2.0]
  ja: [0.1, 2.0]
  ko: [0.1, 2.0]
  ar: [0.3, 3.0]