refactor: serve the read routes from a ModelSnapshot that loop() fills

/controller, /discovery, /rooms, /shades, /groups, /room, /shade and /group
now take a SnapshotSlots slot, queue a fill on the LoopQueue and send a
chunked response whose body waits (RESPONSE_TRY_AGAIN) until loop() has
copied the shade model into the slot. SnapshotWriter then streams the
document from that copy a unit at a time, so each document describes one
moment. At most two reads run at once; a third gets 503. A slot's 5.6 KB
snapshot is allocated only while a read holds it, and its atomic state
decides whether loop() or the read frees it when the read goes away first.

The JSON of each item is unchanged and now lives in ModelSnapshot.cpp,
where the native tests check it. Replies still written on the web server
task copy their one item with SomfySnapshot.cpp, so serializing a group
no longer calls updateFlags(), which emitted groupState and published MQTT
from async_tcp.

Also: every read route is chunked and leaves out an item too big for its
3 KB unit, like /controller did; /discovery drops its timing log; the
SOMFY_MAX_* limits move to ShadeTypes.h.

21 new native tests, 292 in all. Flash: esp32devdbg +440 bytes, esp32dev
+2,064 bytes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ShadeTypes.h

@@ -2,8 +2,16 @@
 #define SHADE_TYPES_H
 #include <Arduino.h>
 
-// The kinds of shade, tilt and group.  Kept free of the shade model so the
-// shade maths can use them in the unit tests on the PC (pio test -e native).
+// The kinds of shade, tilt and group, and how many of each the shade model
+// holds.  Kept free of the shade model so the shade maths can use them in the
+// unit tests on the PC (pio test -e native).
+
+#define SOMFY_MAX_SHADES 32
+#define SOMFY_MAX_GROUPS 16
+#define SOMFY_MAX_LINKED_REMOTES 7
+#define SOMFY_MAX_GROUPED_SHADES 32
+#define SOMFY_MAX_ROOMS 16
+#define SOMFY_MAX_REPEATERS 7
 
 enum class group_types : byte {
   channel = 0x00