fix(transcript): reserve a stable slot for tail runtime status

[vastsa]

Summary

The transcript's tail status row ("waiting for model", planning, working) was conditionally rendered straight into .thread-content, so the row's line box, padding and margin entered and left the normal document flow with every phase change. While the user is pinned to the bottom, the content ResizeObserver in useTranscriptScroll.ts re-baselines follow synchronously, so the height change moved rows the user was already reading. Reported and measured at 46 CSS px of heading displacement on Windows/Electron 43.6.0 at DPR=1, with no manual disclosure and no user scrolling involved (that separate path is #324).

Changes

apps/desktop/src/features/chat/transcript/ChatTranscript.tsx

• The three tail indicators are wrapped in an always-mounted .transcript-runtime-status lane, gated on runtimeStatusLane (== transcriptRunning). All three indicators already required transcriptRunning, so the gate cannot hide one: only the wrapper's lifetime changed.
• The indicators keep their exact conditional expressions on a single line, and the lane's slot is reserved for the whole running turn.

apps/desktop/src/styles/chat-shell.css

• .transcript-runtime-status reserves exactly one indicator box: calc(var(--text-sm-plus) * var(--leading-body) + 22px) — the --text-sm-plus line at --leading-body plus the indicator's own 8px/4px padding and 2px/8px margins, which is why display: flow-root is needed so an empty lane does not collapse its margins away. Measured reserve: 40.125 px, the indicator's own height.
• An idle transcript (turn over, or reading window) renders no lane at all, so a finished transcript keeps its exact layout.

No scroll compensation, no timeout-based re-anchoring, and the ResizeObserver, overflow-anchor settings, pinned-follow rule and automatic disclosure rules are untouched.

Validation

On the integrated local main (merge commit b1bd0f2b, after merging this branch):

Command	Result
pnpm build:js	pass
pnpm --filter @pi-desktop/desktop typecheck	pass
pnpm lint:biome	pass
pnpm -r --if-present test	pass — desktop 1979/1979, 0 fail
pnpm test:e2e:transcript	pass
pnpm test:e2e:theme-surfaces	pass, failures: []
pnpm test:e2e:layout	pass, 37/37 checks
pnpm test:e2e:boot	pass

The new scenario E2E-CHAT-runtime-status-keeps-row-position measures the real ChatTranscript in a real Electron viewport with the app's built stylesheet linked in (the previous fixture bundled components with loader: { ".css": "empty" }, so a CSS-reserved slot was not measurable — the harness now links the built CSS the same way test:e2e:theme-surfaces already does, and asserts the linked stylesheet list matches the built one so a stale build cannot pass).

Geometry is byte-identical across empty lane → waiting → cleared:

contentHeight 1119.9375  scrollTop 548  firstRowTop -502  lastRowBottom 287.8125
laneHeight    40.125     laneChildren 0 -> 1 -> 0     laneText "" -> "Waiting for model0s" -> ""
idleLaneMounted: false    failures: []

Removing the min-height and rebuilding makes the scenario fail with exactly the reported symptom (contentHeight moved by 40.125px), so the test is not vacuous.

Specs

• docs/spec/04-ux/08-component-spec.md — the tail status lane contract.
• docs/spec/06-delivery/04-e2e-test-plan.md — E2E-CHAT-runtime-status-keeps-row-position.
• Renderer only: no protocol, storage, host, permission, migration or IPC change.

Known boundary

The lane's reserve starts and ends with the turn (transcriptRunning), and entering/leaving the reading window removes it. Both are real content changes the transcript already follows; the fix targets the intra-turn flicker the issue reports. A one-line reserve can wrap in an extremely narrow pane; the measured scenario is the issue's own width regime.

Fixes #323

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