chore(license): add GPL-3.0-or-later LICENSE + pyproject metadata

DECNET had no LICENSE file and no license metadata in pyproject.toml
despite intent being GPLv3. Legally that meant the code was "all
rights reserved" by default, so anyone distributing it (including via
GitHub clones, mirrors, or the forthcoming swarm enroll bundles) was
technically in violation even though the operator's own intent was
copyleft.

- Add canonical GPL-3.0 text from gnu.org/licenses/gpl-3.0.txt as
  LICENSE (verbatim, 674 lines).
- Add license = "GPL-3.0-or-later" and license-files = ["LICENSE"]
  to pyproject.toml [project] (SPDX identifier per PEP 639).
- Add the matching OSI classifier plus a few other standard ones
  (Python 3.11, Linux, Security, Network Monitoring, Beta) that
  pyproject was silently missing.

Prereq for the forthcoming p0f-db vendoring: establishing DECNET's
own license explicitly closes the first question an auditor would
ask about any third-party data we embed.

pyproject.toml

@@ -6,7 +6,17 @@ build-backend = "setuptools.build_meta"
 name = "decnet"
 version = "0.2.0"
 description = "Deception network: deploy honeypot deckies that appear as real LAN hosts"
+license = "GPL-3.0-or-later"
+license-files = ["LICENSE"]
 requires-python = ">=3.11"
+classifiers = [
+    "License :: OSI Approved :: GNU General Public License v3 or later (GPLv3+)",
+    "Programming Language :: Python :: 3.11",
+    "Operating System :: POSIX :: Linux",
+    "Topic :: Security",
+    "Topic :: System :: Networking :: Monitoring",
+    "Development Status :: 4 - Beta",
+]
 dependencies = [
     "typer[all]>=0.12",
     "pydantic>=2.0",