docs(#195): design spec for four projection read-model origin capabilities

Semantic, backend-agnostic design (Fable-reviewed): origin.aggregate @agg
any/all (predicate quantifier reusing the filter AST) + collect (array rollup);
new origin.computed (@expr structured expression tree, filter-embeddable, one
cross-port eval engine) + origin.first (generalized pick-one-related-row-by-
ordering); shared @orderby semantic primitive. Each origin is a pure derivation
(rows -> value); RDB SQL is one lowering, verified against in-process + document
-store. Naming origin.computed (retires the origin.expression reservation). The
"not migrate-managed" escape valve is split to a follow-up FR (generalizes to any
DB object, ties to an @sqlView-style DDL-carrying attr + a managed-elsewhere marker).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NGQ7oSuNcjhsMHWwZzhBwr

Author: dmealing