docs: entity-level unique: is described in the in-repo contract (#7228)

[delchev]

The gap

#7116 (#7092) lets an entity-level unique: key name a cross-model to-one. The only place that rule - or the construct itself, from #6793/#6796 - was written down is intent-assistant-guide.md. .claude/docs/intent-layer.md mentions only the field attribute unique and the schedule's generate.unique:; engine-intent/CLAUDE.md and the module README never described the composite business key at all.

The change

Docs, plus one assertion.

• .claude/docs/intent-layer.md - a paragraph beside the other entity-level constructs: the shape, the member rules, what is emitted, and why a cross-model member qualifies.
• components/engine/engine-intent/CLAUDE.md - the module-level bullet, next to immutableWhen: / checks:: UniqueIntent + validateUnique's refusals (a to-many, a subset whose column holds a normalized set rather than an identity, a single-name key naming the field attribute it duplicates, a repeated member, a repeated key), and EdmIntentGenerator.uniqueConstraint's output - the <Entity>_<Member> constraint name, the upperSnake(entity)_upperSnake(member) columns, columnsCsv, the properties twin the modeler re-resolves when it rebuilds the .model, the authored-or-generated message, the synthesized (partition, number) key of a partitioned number:. The cross-model half is stated as the reason it needed no resolution step: the column name is derived from the names, and the consumer stores the target's id in its own FK column - the projection entity is only the read-side copy. Ends with where it is covered (IntentCrossModelUniqueIT, and the PartyCode runtime fixture in IntentEmissionCoverageIT).
• IntentCrossModelUniqueIT.cleanup - statusCode(greaterThanOrEqualTo(200)) accepts a 404 and a 500, so a teardown that stopped working would stay green. Now both(greaterThanOrEqualTo(200)).and(lessThan(300)), as in the sibling IntentCrossModelFormFieldIT.

Verification

• mvn -o -pl tests/tests-integrations compile - BUILD SUCCESS (the new both / lessThan imports).
• mvn -o -pl tests/tests-integrations formatter:validate with the formatter cache wiped first - BUILD SUCCESS.
• No production code changed, so no generated output moves.

Fixes #7228

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