Preserve m_pLod for IPL siblings sharing a single LOD entity (#4863)

#### Summary

Stop vanilla `_LinkLods` from nulling `m_pLod` on shared-LOD IPL
siblings. NOP the 7-byte store at `0x5B52F8`.

#### Motivation

When several high-detail buildings reference the same LOD instance
(common for billboards like `BillBd3` 1260 / `LODbillbd03` 1266),
`_LinkLods` clears `m_pLod` on every sibling except the last. The
result is `LOD = 0` on most shared-LOD entities for the rest of the
session, breaking `processLineOfSight` LOD model id, debug overlays,
and any other code that reads `m_pLod` at runtime.

The `numChildren` decrement two instructions earlier is kept, so the
count still drops to 1 and the final sibling still triggers the
per-model collision swap exactly once. Tunnel keep-LOD (`flag 0x20`),
single-child collision sharing, and streamed IPL loads
(`CIplStore::LoadIpl`) are untouched.

Reported by SpeedyFolf.

#### Test plan

Static (IDA, gta_sa.exe 1.0 US, MD5 `667f799c4ba8c9e1054fccaea6d4259b`):

* `m_pLod` offset confirmed at `+0x30` against the IPL parser's own
  write at `0x53835C` (`89 46 30`).
* `_LinkLods` only callable from `CFileLoader::LoadScene` (`0x5B8A7F`).
* Decompile after patch: `else` branch is just the `numChildren`
  decrement, with the `m_pLod = 0` store gone.
* No other `mov dword ptr [reg+30h], 0` site re-introduces the nuller
  (rest are unrelated D3D/RW/libpng, or ctor/sentinel writes).

In-game:

1. Stand near a cluster of `BillBd3` billboards.
2. `processLineOfSight` each and read the LOD model id.
3. Before patch: subset reports `0`, rest report `1266`.
4. After patch: all report `1266`.

#### Checklist

* [x] Your code should follow the [coding
guidelines](https://wiki.multitheftauto.com/index.php?title=Coding_guidelines).
* [x] Smaller pull requests are easier to review. If your pull request
is beefy, your pull request should be reviewable commit-by-commit.

Author: Zephkek

Client/game_sa/CFileLoaderSA.cpp

@@ -26,6 +26,20 @@ void CFileLoaderSA::StaticSetHooks()
     HookInstall(0x5371F0, (DWORD)CFileLoader_LoadAtomicFile, 5);
     HookInstall(0x537150, (DWORD)CFileLoader_SetRelatedModelInfoCB, 5);
     HookInstall(0x538690, (DWORD)CFileLoader_LoadObjectInstance, 5);
+
+    // Preserve m_pLod for buildings sharing one LOD entity at scene-load time.
+    // Vanilla _LinkLods (0x5B51E0) walks the IPL instance list and, when several
+    // high-detail buildings reference the same LOD instance, deregisters all but
+    // the last sibling by writing 0 to their m_pLod field at 0x5B52F8
+    // (mov dword ptr [esi+30h], 0). The decrement of the LOD's numChildren a few
+    // bytes earlier is what brings the count down to 1 so the final sibling can
+    // pick up the shared-collision path; that is intentional and is left intact.
+    // Nulling m_pLod is order-dependent and visibly strands many shared-LOD
+    // billboards (e.g. BillBd3 / model 1260) with no LOD reference at runtime,
+    // which breaks LOD-aware lookups (collision queries, processLineOfSight LOD
+    // model id, etc.). NOPing the 7-byte store keeps every sibling linked while
+    // the per-model collision swap still happens exactly once.
+    MemSet((void*)0x5B52F8, 0x90, 7);
 }
 
 CEntitySAInterface* CFileLoaderSA::LoadObjectInstance(SFileObjectInstance* obj)