docs: add security policy (SECURITY.md)

.github/SECURITY.md

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+# Security Policy
+
+## Reporting a Vulnerability
+
+If you discover a security vulnerability in BetterCast, please report it
+privately so it can be addressed before public disclosure.
+
+- **Email:** hi@stephenlovino.com
+- Please use the subject line `BetterCast Security` and include:
+  - A description of the vulnerability and its potential impact
+  - Steps to reproduce (proof-of-concept if possible)
+  - The affected version, platform, and connection mode (Wi-Fi or USB)
+
+Please do **not** open a public GitHub issue for security vulnerabilities.
+
+## What to Expect
+
+- We aim to acknowledge your report within **72 hours**.
+- We will keep you updated as we investigate and work on a fix.
+- Once a fix is released, we are happy to credit you for the disclosure
+  (let us know if you would prefer to remain anonymous).
+
+## Scope
+
+BetterCast streams display content directly between your own devices over
+your local network (Wi-Fi or USB-C). It does not use relay servers, cloud
+services, accounts, or telemetry, and it does not collect personal data.
+Reports most relevant to this project include, for example:
+
+- Flaws in the peer-to-peer streaming or device-discovery protocol
+- Issues that could let an unauthorized device on the network connect,
+  intercept, or inject display content
+- Memory-safety or input-handling bugs in the sender or receiver apps
+
+## Supported Versions
+
+Security fixes are applied to the latest released version. Please make sure
+you are running the most recent release before reporting an issue.