feat(web-shell): select a time range on the trajectory overview to narrow the table - #12543

What this PR does

Press on the trajectory overview strip and drag sideways to select a stretch of time. The table then keeps only the requests and tool calls that were running at any point in that stretch, ends included, plus the header and prompt of each turn they belong to, so every surviving row still says which turn and which ask it answered. On the strip, the selection is drawn as a light wash with an edge line on each side, and spans outside it fade. The header swaps its totals for "Showing n of m rows in the selected time" and grows a clear button.

The selection can be dropped four ways: the clear button, Escape on the table, a right click on the strip, or a plain click on empty track. A press that travels less than 4 px is still a click. On a span it selects that row exactly as it does today; on a span that is outside the selection, it drops the selection first and then reveals the row. Refreshing the window, or opening another session, drops the selection in the same render, because the compressed time axis is laid out again and the old numbers would name a different stretch.

Why it's needed

Part of #12293. The overview shows where a run spent its time, but it could not answer the obvious next question, "what happened in these two minutes?", without scrolling the table by hand. This follows the range focus in deepseek-harness's trajectory overview: an inclusive range, where a record active at any point in it counts.

This is the first half of the overview's interaction step. Wheel zoom, right-drag panning and a real-clock mode are left to a follow-up, because they are a second, separate piece of pointer geometry, and each such piece has needed real-browser checks of its own.

Reviewer Test Plan

How to verify

1. Open the right panel's Trajectory tab on a session with a few turns. Drag across the overview strip. The table narrows to the turns that ran in that stretch, the header shows the row count, and spans outside the stretch fade.
2. Press Escape on the table, or the header's clear button, or right-click the strip: every row comes back.
3. Click a single span without dragging: its row is selected and scrolled into view, and no selection is made. Click a faded span while a selection is active: the selection is dropped and that row is revealed.
4. The browser specs under "time selection" in web-shell.trajectory.spec.ts drive all of this with real mouse input.

Evidence (Before & After)

Captured by CI's Capture web-shell visuals job on head 02dfeab: the same session as the existing trajectory capture, with a stretch dragged across the middle of the overview using real mouse input. The band sits between two edge lines, spans outside it fade, the table keeps the one turn that ran in that time, and the header reads "Showing 5 of 10 rows in the selected time" beside the clear button.

Dark	Light

web-shell E2E Smoke on the same head: 160 passed, and the log shows all five new cases below executing, not skipped. The export renderer bundle measured 1,942,157 bytes in that run, under the 1,970,000 warning line.

Browser specs added, all @smoke, using real page.mouse input:

• narrows the table to the dragged time: exact row count, header text, faded-span count, and the band's edges within 1.5 px of where the pointer was pressed and released
• a click without a drag selects the span and leaves the table whole
• Escape and the clear button bring every row back
• a right click on the overview clears the selection
• holds the table where it is while it narrows and widens

Unit tests: 146 pass across the trajectory hook, loader, projection, range helper, panel and overview. Fourteen mutations were each killed by at least one test, including an open interval instead of a closed one, no drag threshold, a click on a span committing a range, the drag band left drawn after release, turns kept regardless of the range, the prompt row dropped, the range surviving a refresh, no fading, no clamping at the strip's ends, Escape claimed with no selection, the selected span faded, a cancelled press committing, a press outside the range ignored, and the browser menu left open on right click.

Tested on

Environment (optional)

Unit tests locally on Linux. The browser specs run in CI's web-shell E2E Smoke; Chromium cannot run on the machine these changes were written on.

Risk & Scope

• Main risk or tradeoff: the selection is pointer-only. The keyboard can drop it but cannot make one. Filtering does not change which row is selected; if the selected row is filtered out, the arrow keys start again from the first row.
• Not validated / out of scope: wheel zoom, panning and a real-clock mode are a follow-up. Because idle time is cut out of the axis, spans cover it end to end and any selection finds at least one row, so there is no "nothing in this stretch" state yet; it arrives with the real-clock mode.
• Breaking changes / migration notes: none. Only the Trajectory tab changes, which hosts opt into. No SDK, daemon or protocol change.

Linked Issues

Part of #12293