B5: `=1` + `one` plural pairing produces gen-l10n 'overridden' warnings on most locales

[ChauCM]

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

Problem

The convention tells the agent to include both an exact-match =1 and the CLDR one category in every plural. For most target locales we ship (en/es/de/ja/vi/ar), CLDR one matches only n=1. After =1 matches, one is dead code, and flutter gen-l10n correctly warns:

[app_de.arb:checkoutNights] ICU Syntax Warning: The plural part specified below is overridden by a later plural part.
    {nights, plural, =1{1 Nacht} one{{nights} Nacht} other{...}}

Harmless at runtime, but every adopter sees these on first sync.

Options

• (A) Refine the convention: "If your locale's one covers values beyond 1 (Russian, Polish), include both. If one matches only n=1, prefer =1 and skip one." Most teachable.
• (B) Add dialect check / dialect lint-plurals detection of the overlap + --fix to remove redundant one.
• (C) Document the warning is intentional belt-and-braces.

The original reporter votes (A) with (B) as enforcement.

Acceptance

• dialect.yaml's plural section explicitly distinguishes CLDR-one-only-equals-1 locales from CLDR-one-broader locales.
• dialect check flags redundant =1+one pairs in locales where one matches only 1.