[Buyer Gap] Open project evidence as a unified chronological history timeline #279

Buyer problem

The Buyer surface can already show source_project_code / source_project_name, semantic post_project_mention evidence, Event Lineage, R&R actors, tickets, and post details. However, a project evidence card currently offers only Search related posts. A buyer cannot open one project and inspect the full order-to-current-issue history on one screen.

The required To-Be surface is a project timeline such as:

Order → specification change → delivery → VOC → rebid

with the selected event showing:

• the related order and prior history,
• the relevant specification-change history,
• observed responsibility history,
• responsibility handoffs or intervals with no observed assignee,
• exact source records and provenance.

Root cause

Project identity, chronological post facts, reconstruction edges, and R&R evidence are stored in separate read paths. There is no ABAC-safe project-history projection that joins them without changing their truth status. The frontend therefore falls back to a board text search rather than a project-level history view.

Product boundary

This feature must explain stored evidence without inventing project-management facts.

• PostgreSQL source records remain authoritative.
• source_project_* fields are explicit source hints.
• post_project_mention rows are semantic candidates with their existing confidence and provenance.
• post_lineage_edge is reconstructed/inferred related history, not a causal claim.
• post_summary_role is observed R&R evidence, not an authoritative HR assignment ledger.
• The first release uses source_post.created_at as document_time unless an explicit event clock exists; it must not relabel document time as event time.
• Unauthorized private, draft-excluded, or deletion-excluded posts must not influence the timeline, reverse trace, counts, or handoff analysis.

Functional acceptance criteria

Backend projection

• Add a versioned GET /api/project-history?project_key=... read contract.
• Match authorized posts by exact normalized explicit project code/name or exact stored semantic project key/name.
• Bound the request and response; stable-sort by (occurred_at, post_id) and report truncation.
• Return a declared time_basis_code; default to document_time for the current schema.
• Return typed related-prior paths using only visible project posts and persisted lineage edges.
• Return each path's truth status, source relation, score where applicable, and provenance.
• Return observed R&R actors per event and adjacent-event responsibility transitions: continuous, handoff, or assignment_gap.
• Return the distinct observed actor count without treating missing R&R as a negative assignment fact.

Buyer UI

• Replace the project card's search-only dead end with Open project history while retaining board search as a secondary action.
• Render an accessible chronological timeline with event date, title, and declared time basis.
• Selecting an event updates one detail panel with related prior history and observed responsibility history.
• Handoffs and gaps are communicated by text/icons, not color alone.
• Provide keyboard operation, visible focus, aria-current, responsive reflow, and an exact-value table/ordered-list representation.
• Allow opening the selected event's source post without losing the originating Buyer context.
• Localize the surface for Korean, English, Chinese, Japanese, and Vietnamese.
• Add a Storybook fixture representing order, specification change, delivery, VOC, and rebid.

Required tests

• RED→GREEN API contract tests for exact project matching and stable chronology.
• ABAC regression proving an unauthorized private event cannot affect history, lineage paths, actor counts, or handoff segments.
• Regression for explicit and semantic project evidence converging on one event without duplication.
• Unit tests for reverse-path traversal and continuous / handoff / assignment_gap classification.
• Frontend tests for selection, keyboard navigation, related-history disclosure, responsibility count, and source-post opening.
• I18n and Storybook build checks.
• Full PostgreSQL/Python and frontend lint/test/build gates on the exact head.

Standards grounding

• W3C PROV-O for portable provenance and explicit truth/source boundaries: https://www.w3.org/TR/prov-o/
• W3C OWL-Time for temporal positions and ordering semantics: https://www.w3.org/TR/owl-time/
• ISO 8601-1:2019 with Amendment 1:2022 for interchange date/time representation; the 2026 edition is still a committee draft: https://www.iso.org/standard/70907.html
• WCAG 2.2 / WAI-ARIA 1.2 keyboard and non-color-only interaction requirements: https://www.w3.org/WAI/WCAG22/Understanding/ and https://www.w3.org/TR/wai-aria/

Stack and merge boundary

Implement as a new stacked PR on the stabilized head of #264. Do not mix the separate #274 channel-score evidence-table work into this slice. #258 must ultimately land before the descendant Buyer-surface stack is merged.