feat(schema)!: restrict grouped select to single-value expressions

[kudima03]

Summary

Refs #52. Covers the groupBy side: the schema now rejects a field that isn't grouped when it appears in select of a grouped query.

JSON Schema can't compare the fields in select with the fields in groupBy. So instead of trying to check that, this PR changes the query shape so the invalid case can't be written at all:

• Schema: root dependentSchemas: when groupBy is present, every select item must be singleValueReturning. This works like dependentRequired from fix(schema): require groupBy when having is present #58 and needs no if/then. alias is still checked by the root selectExpression, because dependentSchemas adds to the root schema and doesn't replace it.
• Semantics: the groupBy fields are output automatically as the first result columns, in groupBy order and named after the field. Next come the select entries. This matches LINQ, where .GroupBy(k).Select(g => ...) gives you g.Key plus aggregates over g.
• Samples: removed the group-key fields from select in 07, 08, 12 and 18. Sample 12 loses its aliases on the group keys (customer_id, customer_name, membership_tier).
• Docs: README, CLAUDE.md, and a breaking-change entry in CHANGELOG.md under [Unreleased].

Verification

All 21 samples validate. Scenario checks, using 08_having.json as the base:

Scenario	Result
grouped, aggregates only	valid
grouped, arithmetic over aggregate (multiply(sum, 0.05))	valid
grouped, scalar literal	valid
grouped, group-key field repeated in select	invalid
grouped, field that isn't grouped (status)	invalid
grouped, eachMultiply column	invalid
ungrouped, field + aggregate	valid (broadcast semantics, to be documented separately)

Breaking change

Grouped queries that list any field or each* expression in select no longer validate. To migrate, delete the group-key fields from select; they now appear in the result automatically. The release needs a major bump.

Not in this PR

• The broadcast semantics for a field + aggregate in select without groupBy (the other half of Prevent mixing aggregate and non-aggregate select without groupBy at schema level #52). That's documentation only and will come in a follow-up.
• Aliases on group keys can't be set.

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