Release 0.18.0: bump metadata.json to match VERSION (cross-class clearance)

VERSION was already 0.18.0; align metadata.json version + download_url so the
release preflight (check_release_version.py) passes. Headline feature: pr392
cross-class clearance (--net-clearances, auto-read netclasses, --keep-input-copper).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EnkvfHs8NiTT97BBUWq5qW

Author: drandyhaas

metadata.json

@@ -25,7 +25,7 @@
   ],
   "versions": [
     {
-      "version": "0.17.7",
+      "version": "0.18.0",
       "status": "testing",
       "kicad_version": "9.0",
       "runtime": "swig",
@@ -34,7 +34,7 @@
         "macos",
         "windows"
       ],
-      "download_url": "https://github.com/drandyhaas/KiCadRoutingTools/releases/download/v0.17.7/KiCadRoutingTools-0.17.7.zip",
+      "download_url": "https://github.com/drandyhaas/KiCadRoutingTools/releases/download/v0.18.0/KiCadRoutingTools-0.18.0.zip",
       "download_sha256": "0000000000000000000000000000000000000000000000000000000000000000",
       "download_size": 0,
       "install_size": 0