Refactor pre-aggregation planner into explicit stages

[hussainsultan]

Summary

• move source-aware pre-aggregation out of the large SemanticAggregateOp method into a dedicated planner module
• model eight explicit planning stages and four source-level substages with frozen, slotted attrs records
• retain a single mutable attrs builder for incrementally assembled source products
• add stage-contract and fan-out-safety tests plus an architecture decision record

Design

The existing entry point now delegates to PreAggregationPlanner. Each stage has an explicit input and output contract, while the declared stage_types tuple makes the pipeline order inspectable. The source pre-aggregation stage is further split into preparation, measure resolution, grain planning, and compilation.

Verification

• 1,189 semantic tests passed; 1 skipped; 11 xfailed; 4 xpassed
• 249 focused pre-aggregation and soundness tests passed
• 5 new stage-contract tests passed
• Ruff checks and git diff validation passed