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/Graveyards.json

@@ -1,6 +1,6 @@
-[
+[
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "003c2e17-0f7e-4af8-9b25-de6ccc4c6e24",
     "name": "Spawner (108)",
     "location": [4545, 1317, 8],
@@ -9,7 +9,10 @@
     "minDelay": "00:05:00",
     "maxDelay": "00:10:00",
     "team": 0,
-    "spawnBounds": { "start": { "x": 4535, "y": 1307, "z": -128 }, "end": { "x": 4555, "y": 1327, "z": 25 } },
+    "spawnBounds": {
+      "start": { "x": 4535, "y": 1307, "z": -128 },
+      "end": { "x": 4555, "y": 1327, "z": 25 }
+    },
     "walkingRange": 20,
     "entries": [
       { "name": "Spectre", "maxCount": 1, "probability": 100 },
@@ -20,7 +23,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "18ac21c4-da65-4449-997e-6e3952e53d46",
     "name": "Spawner (108)",
     "location": [722, 1119, 0],
@@ -36,7 +39,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "1b1194dd-0699-4c9e-9cbe-1332a477a90d",
     "name": "Spawner (108)",
     "location": [1369, 1475, 10],
@@ -55,7 +58,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "69dd4fe9-7f47-466e-b290-963f9bf240d8",
     "name": "Spawner (108)",
     "location": [2758, 867, 0],
@@ -64,7 +67,10 @@
     "minDelay": "00:05:00",
     "maxDelay": "00:10:00",
     "team": 0,
-    "spawnBounds": { "start": { "x": 2738, "y": 847, "z": -128 }, "end": { "x": 2778, "y": 887, "z": 15 } },
+    "spawnBounds": {
+      "start": { "x": 2738, "y": 847, "z": -128 },
+      "end": { "x": 2778, "y": 887, "z": 15 }
+    },
     "walkingRange": 20,
     "entries": [
       { "name": "Spectre", "maxCount": 2, "probability": 100 },
@@ -74,7 +80,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "eb0eb81f-7aea-4162-bf8d-1a0e614ecbcd",
     "name": "Spawner (108)",
     "location": [2438, 1100, 8],
@@ -83,7 +89,10 @@
     "minDelay": "00:05:00",
     "maxDelay": "00:10:00",
     "team": 0,
-    "spawnBounds": { "start": { "x": 2428, "y": 1090, "z": -128 }, "end": { "x": 2448, "y": 1110, "z": 30 } },
+    "spawnBounds": {
+      "start": { "x": 2428, "y": 1090, "z": -128 },
+      "end": { "x": 2448, "y": 1110, "z": 30 }
+    },
     "walkingRange": 20,
     "entries": [
       { "name": "Spectre", "maxCount": 1, "probability": 100 },
@@ -95,7 +104,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "f40bd4f4-6ff1-40ba-84c3-296ade50ab51",
     "name": "Spawner (108)",
     "location": [1285, 3731, 0],
@@ -104,7 +113,10 @@
     "minDelay": "00:05:00",
     "maxDelay": "00:10:00",
     "team": 0,
-    "spawnBounds": { "start": { "x": 1275, "y": 3721, "z": -128 }, "end": { "x": 1295, "y": 3741, "z": 15 } },
+    "spawnBounds": {
+      "start": { "x": 1275, "y": 3721, "z": -128 },
+      "end": { "x": 1295, "y": 3741, "z": 15 }
+    },
     "walkingRange": 15,
     "entries": [
       { "name": "Spectre", "maxCount": 2, "probability": 100 },
@@ -115,7 +127,7 @@
     ]
   },
   {
-    "type": "Spawner",
+    "$type": "Spawner",
     "guid": "fd6f9733-1638-485e-a3fd-3d6f9550591e",
     "name": "Spawner (108)",
     "location": [3407, 2652, 48],