docs: webrtc 0.13 MSRV pin rationale and upgrade checklist

- Cargo.toml: record why webrtc is pinned to 0.13 — >=0.14 pulls sdp 0.10 /
  webrtc-util 0.12 using usize::is_multiple_of (needs rustc >=1.87), while
  rustdesk CI builds with Rust 1.75 (sciter i128 ABI pin)
- module-level upgrade checklist in src/webrtc.rs listing the version-coupled
  webrtc-rs internals this transport relies on (SCTP write backpressure,
  64KB message cap, detach() semantics, handler-capture leak cycle,
  Disconnected transience, stats-based is_relayed), all verified against
  webrtc 0.13 / webrtc-data 0.11 / webrtc-sctp 0.12
- send_bytes: document the bounded-backpressure mechanism (128 KiB PendingQueue
  semaphore + cwnd/rwnd cap) and that it is NOT cancel-safe

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

Cargo.toml

@@ -68,6 +68,10 @@ rustls-pki-types = "1.11"
 rustls-native-certs = "0.8"
 webpki-roots = "1.0.4"
 async-recursion = "1.1"
+# Pinned to 0.13: webrtc >=0.14 pulls sdp 0.10 / webrtc-util 0.12, which use
+# usize::is_multiple_of (needs rustc >=1.87), while rustdesk CI builds with Rust 1.75
+# (sciter i128 ABI pin, flutter-build.yml). Bump only after CI's Rust moves past 1.87,
+# and work through the upgrade checklist at the top of src/webrtc.rs first.
 webrtc = { version = "0.13.0", optional = true }
 libloading = "0.8"
 
@@ -81,7 +85,7 @@ protobuf-codegen = { version = "3.7" }
 
 [dev-dependencies]
 clap = "4.5.51"
-webrtc = "0.13.0"
+webrtc = "0.13.0" # keep in lockstep with [dependencies] webrtc (rustc 1.75 pin, see above)
 
 [target.'cfg(target_os = "windows")'.dependencies]
 winapi = { version = "0.3", features = [