refactor(i18n): chrome labels as data — eight in-code tables become one YAML; es/fr complete (#61)

[danielPoloWork]

Problem (#61)

The three-tier language model (RFC-0001 §7) was implemented as eight duplicated hardcoded tables that had already drifted. An output_lang: es deck was a mixed-language deliverable: ⟨… — por verificar⟩ (Spanish) surrounded by "Risk / Ask / Verify before the room" (English fallback). Adding a language was a seven-file code edit — violating the system's own first principle, knowledge is data, not code.

Fix

• New os/localization/labels.yaml — every chrome string per language: the grounding-marker suffix, the redaction mask, the quiz stems, and the deck / sheet / SVG / quiz / facilitation labels. Namespaced per deliverable (core, md, quiz_md, pptx, docx, svg, xlsx, facilitate) so intentional differences survive (the .docx uses title-case "Target", the sheet says "Source / note"). en is the declared fallback; it sits next to the existing localization/regions.yaml.
• New tools/labels.py — a tiny cached stdlib loader. The eight tables (render.VERIFY_LABEL, the redaction-mask ternary, _QUIZ_STEM/_QUIZ_DISC, emit_md.LABELS + _QUIZ_L, emit_pptx/docx/svg/xlsx.LABELS, facilitate.LABELS) are deleted; each _lab helper now delegates to it.
• es/fr completed for every key — the gap is closed rather than dropped.
• New labels-valid gate — every language defines every key the en fallback defines, and nothing the fallback doesn't. Supported-language completeness is enforced by a check, not convention.

Verification

• it/en output byte-identical before/after on 44 captured files: deck-md, quiz-md, infographic-svg, and facilitate prep for all 11 reference manifests, diff'd clean.
• es/fr end-to-end tests: a Spanish deck renders por verificar + Riesgo + Verificar antes de la reunión with no English fallback leaking; same for French.
• Gate teeth: a half-localized language and a key unknown to the fallback both turn labels-valid red; the shipped file passes complete.
• 87/87 tests green.

Closes #61

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