feat: HellDrinx takeaways — conflict blocklist + vehicle path signal

Two narrow additions adopted from a review of HellDrinx Mod Manager
(/tmp/helldrinx-modmanager-PZ-main.zip), per
docs/plans/ (planning conversation, no spec checked in).

A. _IGNORED_FILENAMES in api/diagnostics.py — skip filenames that are
   intended merge points (PZ engine-concatenated or framework hooks)
   from /api/conflicts output. Multiple distinct sha1s for these files
   is by-design, not a conflict. Live cache had 33 providers shipping
   sandbox-options.txt with 31 distinct hashes — those would have been
   31 false-positive conflict rows on any sort spanning them.

B. Path-based Vehicles signal in worker.build_manifest_and_types —
   extended the existing scripts/vehicles[/] check with
   models_x/vehicles/ and models/vehicles/. Catches vehicle mods that
   ship 3D assets without scripts (rare but real); still requires the
   user-build's manifest to be rebuilt before mod_types reflects it.

api/diagnostics.py

@@ -14,6 +14,29 @@ from typing import Dict, List, Tuple
 from mlos_sort import ModInfo
 
 
+# Filenames that legitimately appear across multiple mods because they're
+# intended merge points — either PZ engine-merged at runtime, or framework
+# extension hooks where multiple client mods coexist by design. Multiple
+# distinct sha1s here is by-design, not a conflict, so we skip them in
+# scan_file_conflicts. Sourced from HellDrinx Mod Manager's known false-
+# positive list (backend/services/workshop.cjs); add more here as we
+# encounter them.
+_IGNORED_FILENAMES = {
+    # PZ engine-merged at load time
+    "sandbox-options.txt",
+    "fileguidtable.xml",
+    # Framework extension hooks (designed for many mods to override)
+    "mf_ismoodle.lua",            # MoodleFramework hook
+    "kp_extrabodylocations.lua",  # KP body-locations framework hook
+    "registries.lua",             # damnlib-style registry framework
+    # Specific mod-side merge points
+    "hat_gasmask.xml",
+    "hat_gasmask_nofilter.xml",
+    "sounds_tmrremovemumble.txt",
+    "null.wav",                   # silent placeholder, deliberately shared
+}
+
+
 @dataclass
 class FileConflict:
     rel_path: str
@@ -59,7 +82,14 @@ async def scan_file_conflicts(conn, mods: List[ModInfo]) -> List[FileConflict]:
         idx = order_index.get(mod_id)
         if idx is None:
             continue
-        by_path[r["rel_path"]].append((idx, mod_id, r["sha1"]))
+        rel_path = r["rel_path"]
+        # Skip known intended-merge-point filenames (engine-concatenated or
+        # framework hooks). These produce noisy false positives because
+        # multiple mods adding sandbox vars / framework hooks is by design.
+        basename = rel_path.rsplit("/", 1)[-1]
+        if basename in _IGNORED_FILENAMES:
+            continue
+        by_path[rel_path].append((idx, mod_id, r["sha1"]))
 
     conflicts: List[FileConflict] = []
     for rel, entries in by_path.items():