docs: add the .NET/C# porting plan

Proposal for replacing the C++ core, C++ server, and Swift macOS/iOS
clients with a single .NET 10 / C# codebase.

Covers the dependency map (8 vcpkg deps + 1 vendored -> 3 native libs),
the real-time-audio design, per-client strategy, a test-porting plan for
all 29 ctest cases, doc-sync work, an 11-phase migration, and a risk
register.

Two findings drive the shape of the plan:

- SslStream has no RFC 5705 keying-material exporter, which the media
  AEAD key derivation depends on (docs/security.md 2). The API is an
  unapproved proposal and SChannel structurally cannot export secrets.
  Recommends BouncyCastle's managed TLS 1.3 stack, which does implement
  the exporter and keeps the wire format byte-compatible with the C++
  implementation -- so the existing tree stays usable as a conformance
  oracle throughout the port. Protocol-v3 in-band media keys documented
  as the fallback.

- The iOS ReplayKit broadcast upload extension stays in Swift: 50 MB
  jetsam cap plus an unsupported extension type in .NET for iOS, and it
  already doesn't link the core. Leaves one Swift file plus the shared
  App Group ring.

Indexed in docs/README.md. Nothing here is implemented yet.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/README.md

@@ -34,6 +34,7 @@ that implementation can start from a shared, agreed plan.
 5. [tech-stack.md](tech-stack.md) — Concrete libraries with versions and rationale, the permissive-license rule, build tooling, per-platform notes.
 6. [deployment.md](deployment.md) — The "set it up in a few minutes" story: Docker, single binary, source build, zero-config defaults.
 7. [roadmap.md](roadmap.md) — Milestones, what ships when, and the list of open questions still to resolve.
+8. [porting-to-dotnet.md](porting-to-dotnet.md) — **Proposal.** Step-by-step plan to replace the C++ core, C++ server, and Swift clients with a single .NET 10 / C# codebase. Dependency map, the TLS-exporter blocker, real-time-audio design, phased migration.
 
 ## Design principles