fix(calendar): lay out overlapping events in side-by-side lanes

[h4yfans]

Summary

Fix day and week view rendering so overlapping events get dedicated lanes instead of stacking at full column width with identical z-index.

The bug: when two events overlapped in time, the later DOM sibling captured every click across the whole column — the earlier chip was visually peeking above/below but completely unclickable for real users. E2E tests had been working around this with `.first()` and `{ force: true }`.

The fix: standard interval graph coloring / greedy lane packing. Sort events by start (longer-first as tiebreak), greedy-assign each to the first lane whose last event has already ended. Within each overlap cluster, render at `width = 100/laneCount` with a lane-indexed `left` offset. Single events still render at full column width — no regression for the common case.

What's in the diff

• `overlap-layout.ts` (new): pure `assignLanes()` with 11 unit tests covering empty, single, non-overlapping, boundary-touching, two-overlap, chain overlap, multi-cluster, three-way, null-endAt, input-order stability, and nested events.
• `calendar-day-view.tsx`, `calendar-week-view.tsx`: wired to `assignLanes` with percentage-based `left`/`width` and `px-0.5` (`@xl:px-1`) inner padding for lane gutters.
• `test-hooks.ts`: optional `overlapMemryTitle` field on `CalendarProjectionSeedInput` that seeds a 9:00–10:00 Memry event overlapping the existing 9:30–10:30 external seed. Backwards compatible — existing callers that omit the field see zero behavior change.
• `calendar-overlap.e2e.ts` (new): 2 Playwright tests covering day and week views. Each asserts chips have disjoint horizontal bounding boxes; the day-view test additionally verifies clicking the external chip opens the promote-external dialog with a real Playwright click (no `dispatchEvent`, no `{ force: true }`, no `.first()`).

Why this approach

Column lane packing is what Google Calendar, Apple Calendar, and Outlook all do — it's the standard UX. The alternative (stack indicator with "+N events" popover) would be inferior on busy days because titles become invisible at a glance.

Cluster-wide `laneCount` (rather than per-event widths based on personal overlap set) keeps the column grid visually consistent — chips align vertically like a mini CSS grid instead of jumping widths.

Test plan

• `vitest run overlap-layout.test.ts` — 11/11 passing
• `vitest run` (full desktop suite) — 7123/7124 passing (1 pre-existing skip, no new failures)
• `typecheck:node` — clean
• `typecheck:web` — clean
• `pnpm lint` — 0 new warnings
• `playwright test calendar-overlap.e2e.ts` — 2/2 passing
• Manual repro: create a 9:00 Memry event that overlaps the 9:30 imported event, confirm both are independently clickable in day and week views

Notes for the reviewer

• Pre-existing failures (not mine). Running the full comprehensive e2e surfaced 5 failures in `Event creation — full editor` / `Event editing` where the "New Event" drawer doesn't open in the built bundle. I stashed my changes, rebuilt, and reproduced the same 5 failures on the branch baseline — they're unrelated to this PR.