Optimize: Make Join interceptors noops in PrebuiltDispatch chains

[DJGosnell]

Summary

• Closes Optimize: Make Join interceptors noops in fully analyzed (PrebuiltDispatch) chains #6
• Add AsJoined<T>() methods to QueryBuilder<T>, JoinedQueryBuilder<T1,T2>, and JoinedQueryBuilder3<T1,T2,T3> that perform type-only conversion without state mutation
• Update InterceptorCodeGenerator to emit AsJoined<T>() instead of AddJoinClause<T>(...) for PrebuiltDispatch chains
• Add 10 unit tests covering all three builder arities

Reason for Change

In fully analyzed (PrebuiltDispatch) chains, AddJoinClause<T>() performs unnecessary work: creating a JoinClause struct, cloning QueryState via ImmutableArray.Add, and setting FromTableAlias. None of this state is consumed by ExecuteWithPrebuiltSqlAsync, which uses a hardcoded SQL const string. Other chain steps (Where, Select) are already effectively noops in this path; Join was the only step doing unnecessary mutation.

Impact

• Eliminates per-query allocation of JoinClause structs and ImmutableArray copies in prebuilt join chains
• Zero behavioral change: prebuilt SQL path never reads JoinClauses or FromTableAlias
• All 2,826 existing tests pass unchanged

Migration Steps

None — internal API only, no consumer-facing changes.

Performance Considerations

Reduces allocations in the hot path for prebuilt join queries. Each join previously allocated a JoinClause struct and cloned QueryState with ImmutableArray.Add; now it's a single constructor call with state passthrough.

Security Considerations

None.

Breaking Changes

• Consumer-facing: None
• Internal: GenerateJoinInterceptor now accepts optional PrebuiltChainInfo? and bool isFirstInChain parameters (source generator internals only)

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