feat(desktop): live command output + agent-produced media (R4)

[physercoe]

Last wedge of vision-parity W3. Consumes E3 (#563) and E4 (#564): a running command's output now appears while it runs, and an image a tool returns renders as an image.

What was actually broken

E3's streamed output was already reaching the client and being dropped by the renderer. useToolMaps folds tool_call_update onto its parent call (latest-wins on toolCallId), but ToolCallBody took only (p, result) — the folded update had no way in — and isToolCallUpdateHidden suppresses the standalone card whenever a visible parent exists. So the output had nowhere to render on either path.

ToolResultBody did str(content) ?? jsonText(content), so an image-bearing result printed a screen of base64.

And blob: refs were skipped with a comment calling them "a future hub". payload_externalize.go already swaps every string leaf over 64 KiB for a blob:sha256/ ref on ingest (handlers_agent_events.go:118) — so for any real screenshot the blob path is the normal one, and images were vanishing at exactly the size where they mattered.

Three of the plan line's five specifics were wrong

Recorded in the plan, because the corrections are the wedge:

1. "fold via streamingPartials.ts" — no. That module is in ui/ not state/, its FOLD_KINDS is text|thought|plan, and its chains key on message_id; a tool_call_update carries toolCallId and no message_id. Nothing needed folding.
2. "MCP image blocks" — only half. claude M2 forwards claude's own tool_result content verbatim (driver_stdio.go:446), i.e. the Anthropic dialect source:{type:"base64",media_type,data}. Measured by running claude --print --output-format stream-json over a PNG rather than recalled — it returns a LIST, with is_error absent rather than false. Both dialects render.
3. "termipod-att:// refs" — a category error. Tauri-only note plumbing for the Read surface (state/attachments.ts:200); no agent event has ever carried one.

Two consequences the plan could not have anticipated, both silent-failure shaped:

• the MIME must come from the event block, since the hub stores an externalized leaf as application/octet-stream — using the blob's mime paints nothing;
• the fetched body is base64 text, so it needs one decode. getBlobDataUrl would double-encode it. No client had ever resolved an externalized payload leaf (mobile's blob code is all artifact viewers, a different case).

A url image source is deliberately not painted — it would make the renderer fetch a host the agent chose.

Also fixed

A pre-existing Companion bug found while threading the new prop: result and callName were handed to the wrong branches, so the dock's call cards folded in nothing and every standalone result was labelled "Result". The full transcript surface was always correct.

Verification

• 17 new pure-module tests; fixtures are captured producer output, not invented shapes.
• 9 mutations checked, all killed. One survived first and exposed a test passing for the wrong reason — a url source was being rejected for carrying no data, not by the source-type guard. Now pinned by a block claiming both.
• 876 frontend tests, tsc, vite build, token ratchet (from root), and the full doc lint set incl. openapi — all green.

Not visually verified — no display on this machine, and the E2E suite does not exercise the transcript. Adds to the standing render debt (#166).

⚠️ Touches the same plan status block as #563/#564 did, so expect a doc conflict if another vision-parity PR lands first.