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.

decnet_web/src/components/RealismConfig/RealismConfig.css

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: AGPL-3.0-or-later */
 /* Realism Config — layered on DeckyFleet.css.
    Adds: weight tables (label + numeric input + percent share),
    canary-class accent, canary-probability slider row. */

decnet_web/src/components/RealismConfig/RealismConfig.tsx

@@ -1,3 +1,4 @@
+// SPDX-License-Identifier: AGPL-3.0-or-later
 import React, { useEffect, useMemo, useState } from 'react';
 import api from '../../utils/api';
 import { useToast } from '../Toasts/useToast';