SPS3: Poly1305-Einmal-MAC + Public-Ready README/LICENSE

Authentifizierung für CLI und Web:
- Poly1305 (RFC 8439) pur in Python (bigint) und JS (BigInt), keine neue Dependency
- MAC-Schluessel pro Nachricht frisch aus dem Pad (len+32), encrypt-then-MAC
  ueber Header + Ciphertext; reveal lehnt manipulierte/desynchrone Bilder hart ab
- Stego-Format SPS2 -> SPS3 (16-B-Tag im Header); alte SPS2-Bilder werden noch
  gelesen, aber als unauthentifiziert markiert
- Service-Worker-Cache auf v3 gebumpt
- Interop-Runner (web/_interop_run.sh): RFC-Vektor cross-impl, beide Richtungen,
  Tamper-Erkennung

README fuer Public ueberarbeitet (ehrliches Bedrohungsmodell, Voraussetzungen,
Projektstruktur, Tests) + MIT-LICENSE.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

LICENSE

@@ -0,0 +1,21 @@
+MIT License
+
+Copyright (c) 2026 Christian Gärtner
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.