Redesign Explore around RecordSet, a unified query model, and manifest-authored presentation

Explore is the owner's window into their own data, but its interaction model is the sum of independently-added parts, not one coherent system. The failures are systemic: a count can promise more records than the UI can reach ("188 upcoming" surfaces only 32); collapse/expand/load-more were not designed as one state machine; there are multiple search inputs and a confusing split between facet filters and typed operators with no way to invert a selection; and record cards pick which field to show by brittle field-name/stream-name heuristics, so arbitrary connectors render wrong. Owner feedback (captured in docs/research/explore-experience-feedback-2026-06-21.md) names all of these. A dual-owner strategy assessment (tmp/workstreams/codex-explore-strategy- assessment.md) concluded the diagnosis is right but two load-bearing abstractions must be made explicit before implementation: a canonical RecordSet / reachability contract, and a manifest-authored record presentation recipe. This change makes those explicit and threads the count == reachability invariant through every named set, grounded in prior art (docs/research/explore-query-filter-ia-prior-art-2026-06-21.md, docs/research/explore-feed-interaction-dynamics-prior-art-2026-06-21.md).

Grounded in: docs/research/explore-query-filter-ia-prior-art-2026-06-21.md, docs/research/explore-feed-interaction-dynamics-prior-art-2026-06-21.md, docs/research/explore-future-dated-records-prior-art-2026-06-21.md, docs/research/explore-now-boundary-pinning-prior-art-2026-06-21.md, docs/research/explore-timeline-legibility-stability-validation-2026-06-19.md, docs/research/explore-experience-feedback-2026-06-21.md. Every design choice cites a prior-art exemplar; PDPP-specific inventions are flagged. The auditable interaction-by- interaction mapping (observed pattern → source → PDPP translation → anti-pattern → acceptance check) lives in design-notes/prior-art-interaction-matrix.md.

The Explore surface SHALL treat every shown count as a RecordSet — a named collection with a declared scope (sources, streams, date range, temporal past/future/all, query, field filters, optional group), a declared count kind (exact | lowerbound | notcounted | hidden), and a declared reachability (inline | paginate | drillin | disabled). This applies to the owner console Explore surface and the reference read surfaces that feed it (GET /ref/explore/records merged timeline + upcoming projection, search pools, and the console's day/burst/facet groupings). A count rendered to the owner SHALL be exact AND fully reachable through its declared reachability, OR it SHALL be hidden. A count SHALL NOT promise more records than the UI can reach, and SHALL NOT be reduced to a smaller loaded-window size in order to appear reachable.