docs: reconcile i18n applicability

[ChelseaKR]

Outcome

Resolves the repository's contradictory i18n claims.

The README said Applies, docs/I18N.md said N/A, and ADR 0006 preserved the
older N/A decision. This PR:

• adopts the conservative Applies/deferred disposition pending backlog feat: pure reading-pace forecast module for TBR and series (EXP-04) #17
• defines the single-user and broader-audience decision paths
• names the owner, first localization boundary, source-language rule, and
  fail-closed fallback
• adds ADR 0007, which supersedes only ADR 0006's i18n decision
• preserves ADR 0006's AI Evaluation N/A decision

No locale catalog or translated copy is claimed.

Validation

• portfolio conformance: i18n declaration passes
• ADR log recognized as eight sequential records
• documentation dispositions agree
• git diff --check