docs(handoff): visual evidence for workbench TruncatedCell (#749/#750) (#753)

Add a handoff brief with five screenshots captured by driving the real
vite-preview build with Playwright (mocked API), confirming the
full-content reveal works across the originally reported proposal queue
table, the trace propagation edges, and the trace selection rails — with
the row staying unselected on reveal (stopPropagation holds).

Verification only; no code change. Records the harness gotcha that a
role-by-name query resolves to the row, not the icon trigger.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

docs/handoff/workbench-truncated-cell-visual-evidence-2026-06-14.md

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+# Workbench `TruncatedCell` — visual evidence (2026-06-14)
+
+Visual confirmation that the full-content reveal for truncated table cells
+(PRs **#749** and **#750**) behaves correctly in the real built workbench UI.
+
+These shots were captured by driving the production `vite preview` build with
+Playwright, mocking the workbench API (`{ ok: true, data }` envelope) with data
+shaped to mirror real responses — long opaque ids, `exemplar_corpus…` sources,
+long pipeline stage names, and long prompt excerpts — so the truncation +
+reveal affordance actually triggers. No code changed; this is verification only.
+
+## What `TruncatedCell` provides
+
+Every truncated data cell keeps its compact display but attaches one
+hover/focus-revealed trigger that opens an accessible popover with the **full
+value** (selectable) plus **one-click copy**; long/structured values also offer
+"Open full view" into a modal. The reveal trigger calls `stopPropagation`, so it
+never selects the surrounding row.
+
+Coverage:
+
+- **#749** — columnar tables: proposal queue, eval wrong=0 case ledger,
+  CORE-Logos contents, proposal artifacts.
+- **#750** — trace propagation edges + every single-column selection rail
+  (trace stage/turns, runs, replay, packs, logos, contemplation, demos) and
+  proposal detail/chain hashes.
+
+## Proposal queue (the originally reported table)
+
+Cognition Queue, accepted filter — truncated `proposal_id` and
+`exemplar_corpus…` source cells, mirroring the reported screenshot.
+
+![Proposal queue table with truncated id and source cells](assets/workbench-truncated-cell/proposals_table.png)
+
+Clicking a cell's reveal icon opens the popover with the **full** `proposal_id`
+(`6ea18d4e5e9f1c2a3b4c5d6e7f8a9b0c1d2e3f40`) and a Copy button. The row stays
+unselected (right panel still shows the placeholder) — `stopPropagation` holds.
+
+![Proposal id cell reveal popover with full value and copy](assets/workbench-truncated-cell/proposals_reveal.png)
+
+## Trace route — edges + selection rails (#750)
+
+Full Trace view: the truncated **Turn Timeline rail** (left), the **stage rail**
+with long stage names (middle), and the **Propagation edges** table (right).
+
+![Trace route showing turns rail, stage rail, and propagation edges](assets/workbench-truncated-cell/trace_full.png)
+
+Reveal on a propagation **edge** — the right-aligned `to_stage` cell
+(`align="end"`) opens a popover with the full long stage name + copy.
+
+![Propagation edge cell reveal popover](assets/workbench-truncated-cell/trace_edge_reveal.png)
+
+Reveal on the **selection rail** — a Turn Timeline row's prompt opens a popover
+with the full multi-line prompt text + copy, without selecting the row.
+
+![Turn timeline rail cell reveal popover with full prompt](assets/workbench-truncated-cell/trace_rail_reveal.png)
+
+## Verification notes
+
+- Built from `origin/main` after #749 + #750 merged.
+- Popovers reported `data-state: open`; rail/edge reveals left the row
+  unselected (`aria-current` count 0) — matches the `stopPropagation` unit test.
+- The reveal trigger is a 16×16 icon button (`button[aria-label^="Show full"]`),
+  distinct from the row's own click target.
+- One harness gotcha worth recording: `getByRole("button", { name: /show full …/ })`
+  resolves to the **row** (its computed accessible name absorbs the child
+  trigger's `aria-label`), not the icon — target the trigger by attribute
+  (`button[aria-label="Show full …"]`) when driving these cells in tests.