fix: add Cross.toml to install libssl-dev for Linux cross-compilation

pforge nightly Linux build fails with:
  Could not find openssl via pkg-config
  The system library `openssl` required by crate `openssl-sys` was not found

Root cause: openssl-sys is pulled transitively via reqwest (default-tls →
native-tls → openssl-sys) and pmcp's websocket (tokio-tungstenite).
The `cross` Docker image used for cross-compilation doesn't have libssl-dev
or pkg-config preinstalled.

Fix: add Cross.toml with pre-build step to install libssl-dev + pkg-config
in the cross container before build.

Alternative (deferred): switch reqwest to rustls-tls and patch pmcp to
avoid native-tls entirely. More work, no binary-size benefit given
trueno-db also pulls openssl indirectly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: noahgift

Cross.toml

@@ -0,0 +1,7 @@
+[build.env]
+passthrough = ["CARGO_TERM_COLOR"]
+
+[target.x86_64-unknown-linux-gnu]
+pre-build = [
+    "apt-get update && apt-get install -y libssl-dev pkg-config"
+]