docs: Stage 3 cost model design doc

Scope of the Stage 3 planning model for designers: what is priced, the
per-second basis for ingestion-time and query-time nodes, recurrence and
horizon, the memory term (1.25e-7 cost/(byte·s), 1 GB ≈ 1/8 vCPU) and its
configuration, illustrative default statistics, interaction with the tree
DP and shared nodes (one node, one computation; "not materialized" is
Stage 2 duplicating the node), a worked Example 1 table, and what is out
of scope. Linked from the proposals README and #509 Stage 3. The Example 1
acceptance doc's costs are restated per second.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: zzylol

docs/design_docs/proposals/README.md

@@ -11,3 +11,4 @@ extensions. A design document is not a promise of downstream runtime support.
 - [Operator sharing](operator-sharing.md)
 - [Decoupling operators from scalar expressions](decoupling_op_and_expr.md)
 - [ASAPPlanner layering](planner-layering.md)
+- [Stage 3 cost model](stage3-cost-model.md)