Give the Debugger Live observation pane one identity line and frame timing

Traceability

• Spec ID: studio-debugger-live-inspector-density
• Status: Proposed

Intent

The Debugger's live right pane stacks four regions before the first piece of protocol evidence: a pane header (Live observation + status), a runtime meta row (project label, Agent label, ACP v1 · Harness stream), a five-row Observed state fact list, and a bordered Raw ACP list. Three concrete problems, not only density:

1. The protocol is named three times and timed zero times. ACP v1 · Harness stream, the ACP frames counter, and the Raw ACP section title all say the same thing, while nothing says when those frames arrived. The pane can report 17 frames without revealing whether they streamed over 200ms or over four minutes.
2. The runtime meta row is chrome that disappears when it is needed. Agent identity, protocol version, and the run's bound Project sit in a floating row between the pane header and its first section, and two media queries hide that row entirely at 1320px and 1080px — dropping the only statement of what this pane is observing exactly where space is tightest.
3. Counters occupy the same visual weight as identifiers. Tool calls, Warnings, and ACP frames are three single-digit numbers, each given a full-width bordered / row equal in weight to a 30-character run id — which itself wraps mid-token because the term column takes 38%.

The Raw ACP list then draws a box border inside a section that already draws its own bottom hairline, and inside a section inset that already indents it.

This change folds the pane's identity into the section it describes, compresses the three counters into one row, gives each identifier its own full-width line, drops the repeated borders, and adds observed timing — the elapsed time between each adjacent pair of frames, plus the total span above the list.

Separately, the live Execution Tree reserves a disclosure column (.tree-caret-spacer, --space-xl) on all four of its rows even though none of them is expandable, so its first node sits 24px inside its own pane header.

Non-Goals

• Changing the recorded (saved-run) StateInspector. Its tabs, comparison line, and Raw ACP dl stay as they are; only the shared .inspector-section and .acp-protocol-list border rules it inherits are touched.
• Dropping the run's bound Project label. It is not a third copy of the sidebar's Project: a sidebar switch during a live run leaves this the only place naming what the run is actually executing in, which test/browser/project-shell.spec.mjs already pins.
• Making live tree rows selectable. They remain non-navigable in live mode; only the dead disclosure column is removed.
• Adding a wire timestamp to HarnessProtocolEvent. Timing is the Studio's own observation clock, read when a frame is folded into run state, and is labelled as observed rather than as agent-reported.
• Persisting frame timing into saved run records.

Acceptance Scenarios

• AC-1: The live Execution Tree reserves no disclosure column. Its first node aligns with its pane header text instead of being indented past an empty caret slot, and depth indentation for turn, prompt, and stage rows is preserved.
• AC-2: The live pane names the observed Agent, protocol, and bound Project exactly once, on a single wrapping identity line that opens the Observed state section. No separate runtime meta row exists above the section, and the identity is never hidden by a width breakpoint.
• AC-3: The bound Project stays named after the sidebar switches to another Project, so an in-flight run cannot be misread as belonging to the Project now selected in the sidebar.
• AC-4: Tool calls, Warnings, and ACP frames render as one row of inline counters, each a value with its own visible label, not as three full-width bordered fact rows. ACP frames still appears only for ACP runs.
• AC-5: Run ID and Thread ID stay visible and complete, each as a monospace value under its own label so a thread_<stamp> id reads on one line at the pane's default width instead of breaking mid-token.
• AC-6: The Raw ACP frame list draws no outer box border and no bottom border under the last section. Adjacent frames stay separated by a single hairline.
• AC-7: Above the frame list, a bounded strip states the first and latest observed frame clock times and the total span between them. It is absent while no frame has been observed.
• AC-8: Each listed frame states the elapsed time since the previous retained frame. The first frame of a run states no elapsed time rather than 0 ms, and a frame whose predecessor scrolled out of the rendered window still reports its true delta.
• AC-9: Timing values use tabular figures and never widen the pane: the live pane stays bounded with no horizontal document overflow at wide (1440), compact (1024), and narrow (390) layouts.

Plan

• app/run/acp-frame-timing.ts (new): pure, exported, and independently testable. acpFrameTimings(events) maps retained frames to \{ observedAt, sincePreviousMs? \} over the full array; acpObservationSpan reduces those to \{ firstAt, lastAt, totalMs \}; formatObservedClock and formatObservedElapsed produce the display strings.
• app/run/run-store.ts: protocolEvents becomes ObservedProtocolEvent[] (HarnessProtocolEvent & \{ observedAt: number \}), stamped from the Studio clock as the protocol-event case folds. The reducer keeps its two-parameter signature — applyHarnessRunEvent is used as a reduce callback, so a clock cannot be added as a third positional argument.
• app/run/RunView.tsx LiveInspector: the .debugger-runtime-meta row becomes a .live-observation-identity line inside the Observed state section, keeping the .debugger-run-project hook; the counters become .observed-counters; the fact list keeps only the two ids as .fact-list-ids; the Raw ACP section delegates to a new AcpFrameList that renders .acp-observation-span above the list and a per-frame .acp-frame-delta inside each summary.
• styles/workbench.css: .live-tree drops the caret column; add .live-observation-identity, .observed-counters, .fact-list-ids, .acp-observation-span, .acp-frame-delta; remove the .acp-protocol-list box border and its section inset; add .inspector-section:last-child \{ border-bottom: 0 \}; delete the two .debugger-runtime-meta media-query rules and its remaining base rules, which no markup targets once the row is gone.
• i18n (en + zh-CN): add raw.frameWindow, raw.firstFrame, raw.latestFrame, raw.totalSpan, raw.sincePrevious.

Test & Review Evidence

• packages/harness-studio/test/acp-frame-timing.test.ts (new, 13 assertions): pairwise deltas over the full array, absent delta for the first frame, a clamped backwards clock, span reduction for zero/one/many frames, and the ms/s/m s formatting boundaries. Each calls a function and checks its returned value.
• packages/harness-studio/test/run-store.test.ts: a folded protocol-event carries a finite observedAt, and a replayed sequence neither appends nor restamps it.
• packages/harness-studio/test/browser/acp-session-stream.spec.mjs: after a streamed ACP run, asserts three counters with ACP frames last, two complete id rows, a two-clock window strip with a numeric span, min(retained, 12) rendered frames, a +<duration> delta on the second frame, no delta on the run's first frame (and a real delta when the tail's predecessor is off-screen), a computed border-top-width: 0px on the frame list, a computed border-bottom-width: 0px on the last section, and a hidden live-tree caret spacer. It also asserts document.documentElement.scrollWidth equals the viewport width at 1440/1024/390 with page errors empty (AC-9).
• packages/harness-studio/test/browser/acp-debugger.spec.mjs: the two Agent identity assertions now target .live-observation-identity.
• Verified in an isolated checkout of exactly this change, with the concurrent ACP session-control work in the tree excluded: packages/harness typechecks, harness-studio builds, vitest run is 584/584, and acp-session-stream + acp-debugger + project-shell browser specs are 18 passed / 1 skipped (macOS NSXPC). This is what proves the run-store change is consistent with the committed harness event union rather than only with the working tree's.
• tool-call.spec.mjs:697 fails on an arrow-key focus assertion in .studio-primary-nav. Reproduced at plain HEAD (1eea73a) in the same isolated setup with none of this change applied, so it is pre-existing and out of scope here.
• In the full working tree (this change plus the concurrent ACP session-control work) the browser suite is 80 passed / 4 failed. Besides tool-call.spec.mjs, acp-debugger.spec.mjs:422, acp-session-stream.spec.mjs:38, and git-history.spec.mjs:98 each pass when run individually and are therefore order-dependent in that tree, not results of this change.

Risk

• Deltas are Studio receipt times, not wire times. Two frames delivered in one SSE flush can read as 0 ms apart when the Agent produced them further apart. The strip is therefore labelled as observed frame timing, and the run's own reported duration in result.metrics remains the authority for run cost.
• Removing .debugger-runtime-meta from this pane deletes CSS that two media queries still target. Both rules are removed in the same change; leaving them would be dead code that a future reader would mistake for a live constraint.