Kiosk: schedule client (conditional GET + atomic cache) and engine

Adds api_client support for GET /licenses/{key}/schedule with If-None-Match
and target-payload parsing (version/type/daily/weekly + legacy blocks), an
atomic on-disk schedule cache, a pure schedule engine (half-open blocks,
midnight self-wrap for daily, Sunday->Monday spillover for weekly, invalid
blocks skipped with a warning), and a background ScheduleManager that loads
the cache offline and pulls on boot and every 5 minutes. Adds engine/cache
tests against the shared docs/fixtures, including 304, corrupt cache, and
offline cases.

src/schedule/schedule_engine.h

@@ -0,0 +1,36 @@
+#pragma once
+
+#include "network/api_client.h"
+#include <functional>
+#include <string>
+
+namespace radiantwave::schedule {
+
+// Weekday keys matching the wire format.
+enum class Weekday { Mon = 0, Tue, Wed, Thu, Fri, Sat, Sun };
+
+// The result of evaluating a schedule at a wall-clock instant.
+struct Decision {
+    bool hasBlock = false;
+    api::TargetBlock block; // valid only when hasBlock
+};
+
+// The active Block for `payload` at (minutes, weekday), or none. Implements the
+// contract: half-open [start, end), midnight-crossing Blocks belong to their
+// start day (weekly Sunday spills into Monday), and a weekly mode must also
+// consider the previous day's crossing Block. Invalid Blocks (empty
+// proclamation list / bad range) are skipped and treated as gaps.
+//
+// Wall-clock minutes are passed in, so DST transitions behave naturally (the
+// caller supplies the local wall-clock time in the Kiosk's timezone).
+Decision evaluate(const api::SchedulePayload& payload, int minutes, Weekday weekday,
+                  std::function<void(const std::string&)> warn = nullptr);
+
+// True when `minutes` falls in the half-open [start, end) block, accounting for
+// a midnight-crossing range.
+bool minuteInBlock(int minutes, int startMinutes, int endMinutes);
+
+// Weekday from a 0=Monday..6=Sunday index (clamped).
+Weekday weekdayFromIndex(int index);
+
+} // namespace radiantwave::schedule