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/LoopQueue.h

@@ -22,12 +22,14 @@
 enum class loop_item_type_t : uint8_t {
   shade_request,   // run by ShadeRequests
   restore_shades,  // restore from /shades.tmp, then reboot
-  load_shades      // replace the shades with /shades.tmp
+  load_shades,     // replace the shades with /shades.tmp
+  snapshot_fill    // fill a read's ModelSnapshot
 };
 struct loop_item_t {
   loop_item_type_t type = loop_item_type_t::shade_request;
   shade_request_t request;    // shade_request
   uint8_t restoreParts = 0;   // restore_shades: RESTORE_* bits
+  uint8_t slot = 0;           // snapshot_fill: the SnapshotSlots slot
 };
 
 // Whether the item sends anything on air.