chore: relicense to AGPL-3.0-or-later and add SPDX headers

Replaces LICENSE (GPLv3 -> AGPLv3) and prepends
`SPDX-License-Identifier: AGPL-3.0-or-later` to every source file
across decnet/, decnet_web/, tests/, scripts/, and tools/.

Rationale: closes the GPLv3 ASP loophole so any party operating a
modified DECNET as a network service must offer their modified
source. Personal copyright (Samuel Paschuan) + inbound=outbound
contributions make a future unilateral relicense infeasible.

- LICENSE: full AGPL-3.0 text (gnu.org/licenses/agpl-3.0.txt)
- COPYRIGHT: project copyright notice
- tools/add_spdx_headers.py: idempotent header injector
  (shebang- and PEP 263-aware)

Touches 1565 source files (.py, .ts, .tsx, .js, .jsx, .css, .sh).
No behavior change; comments only.

scripts/bus/pub.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Publish a single event to the local DECNET bus.
 
 Usage: scripts/bus/pub.py <topic> [json-payload] [--type EVENT_TYPE]

scripts/bus/smoke-mutator.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
 # Mutator-family topic smoke test: boots a bus worker, subscribes to
 # `topology.>`, publishes one event per mutation-lifecycle state
 # (enqueued → applying → applied) plus a topology.status transition,

scripts/bus/smoke.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
 # End-to-end bus smoke test: boots a worker, subscribes, publishes,
 # verifies the event lands, tears everything down. Exits non-zero if
 # anything misbehaves.

scripts/bus/start.sh

@@ -1,4 +1,5 @@
 #!/usr/bin/env bash
+# SPDX-License-Identifier: AGPL-3.0-or-later
 # Start a local `decnet bus` worker for manual smoke-testing.
 # Uses /tmp so it works without root and without the `decnet` POSIX group.
 # Usage: scripts/bus/start.sh [heartbeat-seconds]

scripts/bus/sub.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python3
+# SPDX-License-Identifier: AGPL-3.0-or-later
 """Subscribe to a pattern on the local DECNET bus and print events.
 
 Usage: scripts/bus/sub.py 'topology.>'