docs(plan): FR-044 zero rows from a dimension's entity and a default for an empty measure (#411)

Design for two additions to the 1.1 reporting vocabulary, in all five ports:
@spine on object.report (the report's rows come from a dimension's entity, so
a tuple with no fact rows still has a row) and @default on measure.aggregate
and measure.ratio (the integer a measure reads when it would be null).

The plan settles where each attribute lives and what was rejected, the loader
rules (R8, R9, M7, M8) and their error codes, the view SQL per dialect, what a
measure reads in a row with no facts, the derived shape's nullability and the
row type in every port, and filter and sort on the derived fields. The SQL
shapes were executed on Postgres 16, SQLite and MySQL 8.4.

The spec gains the ADR-0023 register amendment (3.2), requirements R8 and R9,
two mapping rows, one acceptance bullet and the parked list. metamodelVersion
stays 1.1. No code changes in this commit.

Author: dmealing