lint: refuse an incoherent dataset measure (aggregate × field type) at authoring time, from the spec's compatibility matrix (lint leg of #16099) #16354

Lint leg of #16099, split by director ruling (decision batch #59, 2026-09-06, "both legs, table in spec").

Blocked-by: #16353 (the AggregationFunction × FieldType compatibility matrix, exported from @objectstack/spec). This card consumes that table; it declares no rows of its own.

Scope

• A packages/lint rule over dataset measures: for each { aggregate, field }, resolve the field's declared type from the object schema lint already loads and refuse the pair when isAggregateCompatibleWithFieldType says no.
• Message names the aggregate, the field, its type, and the accepted types for that aggregate — so the author's fix is visible without reading the spec.
• Sibling to the compile-time leg on No layer refuses an incoherent aggregate / field-type pair — a dataset measure avg over a datetime works on SQLite and errors on Postgres #16099 (dataset-compiler, services): this rule reaches the author earliest; it cannot see a dataset assembled at runtime, which is why both legs exist.
• Tests: the positive control from the card (avg over datetime fires), a negative control (avg over number silent), and sum over percent (fires — the row analytics-service.ts already calls incoherent).

Acceptance

• rule consumes the spec table; no local copy of the rows
• documented in the lint rule reference
• changeset present