D015: Defer bulk content translation to a final pass

Context: Having Fable translate large content (About, case studies, section copy, brain notes) into 6 locales mid-build burns a lot of tokens for little value, the audience reads English anyway, and translation is a build-time script's job, not Fable's.

Decision: Through all build phases, author content in English only. Non-English locales fall back to English key-by-key (already the wired behavior). Run the full content translation as the final step, via the build-time AI script from D007 - Build-time AI Translation (cheap, outside Fable), then per-locale review (D006 - i18n Seven Languages and RTL).

Do NOT touch the i18n scaffold or the small UI-chrome strings already translated in Phase 0 - Foundation (nav, switcher, theme labels), those stay.

Why: token efficiency; keeps Fable focused on building, not translating; English reaches the whole target audience in the meantime; translation stays a deterministic, reviewable build-time artifact.

Trade-offs: non-English visitors see English body content until the final pass, acceptable and reversible. See Roadmap (translation pass added pre-launch).

Confirmed (chose A): Fable never hand-translates in any scenario (context bloat + poor quality), so the token worry is moot regardless. The deferral is purely about translating final copy once instead of re-translating changing copy every phase. An early translation pass can be run on demand at any checkpoint (it's the build-time script, not Fable), but the default is the single end-of-build pass.