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/felucca/Wrong.json

@@ -1,6 +1,6 @@
-[
+[
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "29168b55-1571-4743-9178-01314586cbe5",
     "name": "Spawner (227)",
     "location": [5819, 591, 0],
@@ -16,7 +16,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "346ff700-516b-4788-85a2-9ee7146726f9",
     "name": "Spawner (227)",
     "location": [5659, 563, 20],
@@ -35,7 +35,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "3ce378dd-3952-4fac-a5fe-ad12a7b09f20",
     "name": "Spawner (227)",
     "location": [5690, 534, 0],
@@ -53,7 +53,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "419487ab-4c4b-4726-8f93-0cd1b49aed0d",
     "name": "Spawner (227)",
     "location": [5808, 588, 12],
@@ -71,7 +71,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "41bd23d3-0a22-41fd-a2d2-5d6d809b2018",
     "name": "Spawner (227)",
     "location": [5828, 530, 0],
@@ -89,7 +89,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "63285a8f-9f0d-4966-9f0e-a7fa33fe3a45",
     "name": "Spawner (227)",
     "location": [5857, 586, 15],
@@ -107,7 +107,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "84d09a21-c001-44ed-9c8a-2e7791fa4b1e",
     "name": "Spawner (227)",
     "location": [5792, 544, 10],
@@ -124,7 +124,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "87ed7838-1bc6-4987-bd1e-0cc16bd54f32",
     "name": "Spawner (227)",
     "location": [5724, 561, 20],
@@ -143,7 +143,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "dfb2dd7e-c751-49e6-9059-25b892efce67",
     "name": "Spawner (227)",
     "location": [5857, 562, 15],