feat(presence): pure presence-logic (status/host-eligibility/summary) + 11 tests

presenceStatus(in,out,now,maxOpenHours) -> present|checked_out|stale
(>=window boundary = stale; unparseable in-time = stale, never present);
hostEligibility honoring the per-space host_requires_card toggle;
summarizePresence counts present + hosts. PRESENCE_MAX_OPEN_HOURS=18.
Suite 421 -> 432.

lib/presence-logic.ts

@@ -0,0 +1,65 @@
+// Pure, dependency-free logic for the presence/attendance feature. No
+// Supabase/React/Next. Unit-tested directly so the "who is present" and
+// host-eligibility rules cannot silently drift.
+
+// An open visit older than this is treated as a forgotten check-out: the
+// member is no longer counted as present and the next check-in auto-closes
+// it. Chosen over a cron job (no background infra, deterministic, testable).
+export const PRESENCE_MAX_OPEN_HOURS = 18
+
+export type PresenceState = 'present' | 'checked_out' | 'stale'
+
+// `present`      = open and within the freshness window (currently here)
+// `checked_out`  = explicitly checked out
+// `stale`        = open but past the window (forgotten check-out; not present)
+export function presenceStatus(
+  checkedInAt: string,
+  checkedOutAt: string | null | undefined,
+  now?: Date | string,
+  maxOpenHours: number = PRESENCE_MAX_OPEN_HOURS,
+): PresenceState {
+  if (checkedOutAt) return 'checked_out'
+  const inMs = new Date(checkedInAt).getTime()
+  if (Number.isNaN(inMs)) return 'stale'
+  const t = now == null ? Date.now() : new Date(now).getTime()
+  const ageHours = (t - inMs) / 3_600_000
+  return ageHours >= maxOpenHours ? 'stale' : 'present'
+}
+
+export type HostEligibility = { ok: true } | { ok: false; reason: string }
+
+// Checking in as a host may require an active access card on file, depending
+// on the per-space `host_requires_card` setting (default true). A plain
+// (non-host) check-in is always allowed.
+export function hostEligibility(input: {
+  asHost: boolean
+  hasActiveCard: boolean
+  hostRequiresCard: boolean
+}): HostEligibility {
+  if (!input.asHost) return { ok: true }
+  if (input.hostRequiresCard && !input.hasActiveCard) {
+    return {
+      ok: false,
+      reason:
+        'Hosting requires an active access card on file. Ask a door manager to add one, or check in without hosting.',
+    }
+  }
+  return { ok: true }
+}
+
+// Count how many visits are currently present, and how many of those are
+// hosting. Stale/checked-out rows are excluded.
+export function summarizePresence(
+  rows: Array<{ checked_in_at: string; checked_out_at: string | null; is_host: boolean }>,
+  now?: Date | string,
+): { present: number; hosts: number } {
+  let present = 0
+  let hosts = 0
+  for (const r of rows) {
+    if (presenceStatus(r.checked_in_at, r.checked_out_at, now) === 'present') {
+      present++
+      if (r.is_host) hosts++
+    }
+  }
+  return { present, hosts }
+}