feat(multi_stage): add median measure type with PERCENTILE_CONT support

[TheRoot-1]

Summary

Adds median as a supported multi_stage measure type, rendered as an ordered-set aggregate (PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY ...)). This is the first half of #10560 — dense_rank is not yet implemented and will come as a follow-up (or separate PR) depending on review feedback.

Without this, users hit a schema validation error (median is not a valid multiStageMeasureType) and have no runtime-filter-respecting way to compute medians — they're forced into static subqueries that ignore filters.

What's included

Schema compiler (JS planner):

• New median type in multiStageMeasureType validator
• SQL rendering path in BaseQuery.js that emits PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY <inner_measure>) against the correct CTE reference
• Unit tests for validator + SQL generation
• Postgres integration test

Tesseract (Rust planner):

• New MultiStageMedianNode sql node
• Registration in SqlNodesFactory
• Measure-kind plumbing in measure_kinds/mod.rs
• Logical + physical plan support (calculation.rs, multi_stage_measure_calculation.rs)
• Member/query planner integration
• 5 integration tests covering basic, group_by, dimension, filter, no-granularity paths
• Unit test on measure symbol properties

Example

measures:
  - name: _inner_order_count
    sql: order_id
    type: count_distinct
    public: false

  - name: median_orders_per_customer
    sql: "{_inner_order_count}"
    type: median
    multi_stage: true
    group_by: []

Generated SQL (Postgres):

WITH cte_0 AS (
  SELECT count(distinct order_id) "cube___inner_order_count" FROM ...
)
SELECT PERCENTILE_CONT(0.5) WITHIN GROUP (ORDER BY "cube___inner_order_count") FROM cte_0

Test plan

• cubejs-schema-compiler unit tests pass (test/unit/base-query.test.ts, test/unit/cube-validator.test.ts)
• cubesqlplanner full suite: 876 passed, 0 failed
• Postgres integration tests (requires running DB — I've written the test at test/integration/postgres/multi-stage.test.ts but haven't run it locally)
• Maintainer sanity check on the SQL shape before widening scope to dense_rank

Notes

• Scoped to median only. dense_rank (the other half of feat(multi_stage): support median (PERCENTILE_CONT) and dense_rank as multi_stage measure types #10560) is intentionally out of scope for this PR; happy to do it as a follow-up once this shape is approved.
• Opening as draft to solicit maintainer feedback before polish.

Closes part of #10560