fix(web): a parallel fork is one line, not a card

[JeremyFunk]

Follow-up to #648, which trimmed the parallel markers but left the shape that made them loud in the first place.

Before

The marker was a bordered, tinted card — headline, prose sentence, a row of jump chips — and then every member row underneath repeated the fact as a ⑂ PARALLEL pill. Three announcements of one thing, in a view whose whole point is that a fork reads at a glance.

After

One rule across the column, in the same shape as the page's other structural lines (empty turn, lane header):

12:24:44   ⑂ 2 LANES IN PARALLEL   overlap 12:24:44 → 12:25:24 ────────────────
12:31:34   ⑂ 3 TURNS IN PARALLEL   overlap 12:31:51 → 12:32:02 ────────────────

• No pills on the subtrees. Turn and lane headers lose them outright — the marker sits directly above its members and the one-lane indentation already says who hangs off it.
• No jump chips. Each member renders whole, in order, right below the marker; there was nowhere to jump to that scrolling did not already reach.
• The chain case still refuses to claim a window every member shared: it says interleaved 12:31:51 → 12:32:02 over the run's extent instead.

What fell out

Nothing reads parallelWith any more, so it goes, and with it the pairwise-overlap filtering that computed it and the overlaps() helper. Also gone: forkedBy (the forking agent's header is the row above the marker), the TurnMarking wrapper (cluster membership is now just a key in the marker map), and the jump plumbing — ParallelJump, ParallelBanner, indexByKey/jumpTo, and onJump off every block's props.

Net −153 lines.

Verification

• tsc clean across apps/web; 283 tests pass in src/lib/agent-sessions + src/components/agent-sessions. Builder tests that asserted chip data now assert marker membership and member indentation instead.
• Both markers rendered and checked against the /lab/agent-session fixture (lane fan-out in turn 15, dispatched turn fan-out in turns 18–20), no console errors.

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