docs/test: model annotations, opt-in enhancements, READMEs

[Schmarvinius]

Scope: pure docs + test resources + opt-in flag. Low risk, ship anytime.

• cds-feature-recommendations:
• Make automatic model-enhancement (adding SAP_Recommendations) opt-in via configuration.
• Document the manual CDS snippet so users can add SAP_Recommendations themselves.
• Provide a slim @cap-js/ai flavor without model artifacts, or exclude the artifacts from cds-feature-recommendations.
• Add a cds add recommendations flavor.
• In README example, ensure entities are draft-enabled.
• Add Fiori-annotations doc links.
• Mark RPT-1-specific config options as such.
• Add Integer64 and clarify UUID in the supported-types table.
• Drop the "Prerequisites" header (it's not really a prereq) and rename ### CDS Plugin properly.
• cds-feature-recommendations/src/test/resources/model/recommendations-test.cds:
• Move @Common.ValueListWithFixedValues from genre_ID to genres association.
• Move @Common.ValueListWithFixedValues from currency_code to currencies association.
• cds-feature-ai-core/README.md:
• Replace "detected automatically via ServiceBindingUtils" — it's an implementation detail.
• Reword cds.requires.AICore (drops once Issue 3 lands).
• Update programmatic-usage snippets to the simplified API once Issue 2 lands.

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