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/network/schedule_cache.h

@@ -0,0 +1,29 @@
+#pragma once
+
+#include "network/api_client.h"
+#include <string>
+
+namespace radiantwave::sync {
+
+// The on-disk schedule cache. Stores the target payload plus its ETag as a JSON
+// document. Writes are atomic (temp file + rename) so a crash or power loss
+// never leaves a partially written cache.
+namespace schedule_cache {
+
+// Absolute path of the cache file (under ~/.local/radiantwave/).
+std::string cachePath();
+
+// Serialize `payload`/`etag` to the cache file atomically. Returns false on
+// any I/O failure; the previous cache is left intact.
+bool write(const api::SchedulePayload& payload, const std::string& etag);
+
+// Load the cache. Returns false (and leaves `out` untouched) when the file is
+// missing, unreadable, malformed, or semantically invalid.
+bool read(api::SchedulePayload& out, std::string& outEtag);
+
+// Remove the cache file. Returns true when absent afterwards.
+bool clear();
+
+} // namespace schedule_cache
+
+} // namespace radiantwave::sync