refactor(spawners): replace DynamicJson with typed SpawnerDto records (#2505)

## Summary

Removes the dated `DynamicJson` JSON helper and migrates spawner JSON (de)serialization to a polymorphic `record SpawnerDto` hierarchy. `DynamicJson` was the last remaining consumer (regions moved off it in #1400).

The key correctness improvement: **System.Text.Json deserializes plain DTO records, never a live `Item`.** Previously, deserializing directly into an `Item` meant STJ constructed world-registered objects *before* the data was validated — a malformed/hand-edited spawn file could leave orphaned spawner Items in the world save. Now a parse failure is GC-only; `dto.ToSpawner()` constructs the spawner only from a fully-validated DTO and self-cleans on failure.

## What changed

- **New:** `SpawnerDto` (abstract) + `SpawnerDataDto` / `RegionSpawnerDto` / `ProximitySpawnerDto`, each marked with a reusable `[JsonDiscoverableType]` opt-in attribute. Auto-discovered at the `Configure` phase — no manual registration list (avoids the regions `Register<T>()` footgun), open to custom spawner subtypes.
- **Symmetric mapping:** `BaseSpawner.ToDto()` (export) ⇄ `SpawnerDto.ToSpawner()` (import). `ToSpawner()` deletes-and-rethrows on any failure, so the importer can never orphan an Item.
- **`SpawnerJsonSerializer`** wires `$type` polymorphism on the `SpawnerDto` root with loud collision/constructibility validation.
- **Import/export commands** rewired to the typed DTO path (reflection `FindTypeByName`/`CreateInstance` removed).
- **Data migration:** the 109 `Distribution/Data/Spawns/**` files moved from `"type"`→`"$type"` and legacy `homeRange`→`spawnBounds`. The runtime still *reads* legacy `homeRange` for external files. The `homeRange→spawnBounds` formula is proven equivalent to the runtime conversion (`BoundsEquivalenceTests`, hr=0/1/3/7).
- **Deleted:** `Projects/Server/Json/DynamicJson.cs`.

Sparse export output matches the legacy `ToJson` (nullable DTO properties + `WhenWritingNull`). Binary world-save serialization is untouched.

## Tests

- DTO round-trip per spawner type; sparse-default omission; legacy `homeRange` read; export/import file round-trip.
- `Import_MalformedFile_LeaksNoWorldItems` — proves a mid-array parse failure constructs zero world Items.
- `AllSpawnFilesLoadTests` — every migrated spawn file deserializes and builds.
- Duplicate-discriminator validation.
- UOContent.Tests 485/485, Server.Tests 710/710, build clean.

## Follow-up (not in this PR)

`Rectangle3DConverter.Write` (in `Projects/Server/`) omits `z1/z2` when `Start.Z == -128`, so a future server-side export of a `homeRange`-style spawner round-trips depth 256→255. Pre-existing and out of scope here (Server change); the migrated data reads correctly. Worth a separate small converter PR.

Distribution/Data/Spawns/shared/trammel/TrinsicPassage.json

@@ -1,6 +1,6 @@
-[
+[
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "01aee8cd-fc2b-4e00-8d21-991131290d9b",
     "name": "Spawner (124)",
     "location": [5999, 1327, 10],
@@ -20,7 +20,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "098f3928-ea97-4278-ad09-63a202b30fea",
     "name": "Spawner (124)",
     "location": [5956, 1310, -2],
@@ -40,7 +40,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "1e21e474-6f3f-4e15-b16a-da201c371cfe",
     "name": "Spawner (124)",
     "location": [5912, 1326, 0],
@@ -60,7 +60,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "54cb95e8-ceb7-42de-872f-6c2208f075c1",
     "name": "Spawner (124)",
     "location": [5976, 1387, -22],
@@ -80,7 +80,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "bdc0fed3-7087-4713-9690-92e8878992cb",
     "name": "Spawner (124)",
     "location": [5940, 1356, -2],
@@ -100,7 +100,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "faefe911-578d-4e50-b1b5-918119689237",
     "name": "Spawner (124)",
     "location": [5915, 1302, 2],