fix: keep the working indicator alive through the empty-first-delta gap

[Zeus-Deus]

Fixes #155

Problem

While an agent run is still in progress, the transcript could read finished and empty: the amber "Working" activity card settled to the green summary, followed by a blank assistant block, with no working indicator anywhere — until prose finally streamed in seconds later. Trigger: the partial-message stream opens a new text content block with an empty first text delta, which materialized a blank assistant_message that (a) settled the live activity run, (b) rendered as nothing, and (c) suppressed the tail marker.

Fix — 4 layers (1 and 2 each independently kill the main symptom)

1. Reducer (src/lib/agent-chat/reducer.ts): appendTextDelta drops an empty/whitespace-only delta instead of creating an empty assistant_message; an empty delta merging into an existing streaming tail is reference-stable. An empty assistant_text completion with nothing to seal no longer lands a blank settled row. The guard is deterministic, so hydrate/replay keeps identical item counts vs live streaming.
2. Marker gate (src/lib/agent-chat/thinking.ts): a streaming-but-empty assistant tail now keeps the tail marker up (defense in depth for providers that still land one).
3. StreamingMarker (src/components/chat/StreamingMarker.tsx): live elapsed-time suffix ("Working… · 40s"). deriveTurnStartedAt anchors to the earliest reducer-stamped started_at after the last non-queued user prompt; the 1s tick writes textContent directly so per-second updates never re-render the transcript. Suffix omitted when no start is derivable (post-send gap, old hydrated transcripts).
4. "Worked through 0 steps" (src/components/chat/activity-steps.ts): the settled-summary fallback now counts other-family tools (Task/agent spawns, MCP tools), so it never claims fewer steps than the header meta.

Invariants preserved: no double indication (marker still suppressed while the tail is a live working Activity block), buildTranscriptSlots untouched and slot keys stable, reducer stays pure with injectable Clock, fix is provider-agnostic at reducer/slot level.

Verification

• tsc --noEmit clean; 2396/2396 tests pass (new coverage in reducer.test.ts, thinking.test.ts, activity-steps.test.ts, new StreamingMarker.test.tsx); cargo check clean (no Rust changes).
• Visual (dev mock, headless browser): streamReply gained { emptyGapTicks } to reproduce the exact stream shape (empty first delta + silence before prose). During the gap the tail stays the amber "Working · 6 done · 0 running" card — never a settled header + blank block — then settles normally once prose lands. The tail marker renders as spinner + shimmer label + mono elapsed suffix.

Docs

• docs/features/agent-chat.md: empty-delta drop, marker elapsed suffix, other-family counting.
• docs/features/dev-mock-runtime.md: emptyGapTicks knob.

Screenshots

Captured against the dev mock (streamReply({ intervalMs: 400, emptyGapTicks: 22 })) in a headless browser.

Mid-gap — all 5 tools done, empty first delta already received. The tail stays the amber "Working · 6 done · 0 running" card. Before this fix it settled to the green summary followed by a blank assistant block:

After prose lands the run settles normally ("Ran commands and inspected the code · 6 steps · 5s") with the streamed text below:

The upgraded tail marker — spinner + shimmer label + mono elapsed suffix ("Working… · 1m 49s"; the large value is a mock artifact — on-demand mock streams have no fresh user prompt, so the anchor is the hydrated transcript's timestamps):