docs(spec): FR-044 core reporting — measures, dimensions, reports and exporters

Requirements for a reporting capability in the core metamodel. Entities
declare dimensions, measures and segments; an object.report names a fixed
combination and compiles to a SQL view through the existing projection
pipeline, so every port can read it. A full semantic layer is delegated to
established tools through reference exporters (Cube first, dbt MetricFlow
second) instead of a MetaObjects query-time engine, which is parked with a
re-entry trigger.

Grounded in a survey of Cube, dbt MetricFlow, LookML, Malloy and lighter
tools, and in an inventory of today's projection vocabulary. All additions
are additive (metamodelVersion 1.0 -> 1.1) and bound by FR-037 R5's
cross-backend admission rule. Six decisions are left open for the
maintainer. Adds the FR-044 roadmap row.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01M1mRRB1K2WdYy82kntMQdx

Author: dmealing