cleanup: add explicit callconv(.c) to all extern fn FFI declarations in ethlibp2p.zig

## Context

PR #743 fixed the x86_64 GPF by adding `extern "C"` to the Rust side of the Zig→Rust FFI boundary. While reviewing that PR, an inconsistency was noticed on the Zig side.

## Issue

In `pkgs/network/src/ethlibp2p.zig`, the `pub extern fn` FFI declarations are inconsistent — some have explicit `callconv(.c)`, others do not:

```zig
// No explicit callconv — relies on extern fn defaulting to C ABI
pub extern fn create_and_run_network(...) void;
pub extern fn wait_for_network_ready(...) bool;
pub extern fn publish_msg_to_rust_bridge(...) void;

// Explicit callconv(.c)
pub extern fn send_rpc_request(...) callconv(.c) u64;
pub extern fn send_rpc_response_chunk(...) callconv(.c) void;
pub extern fn send_rpc_end_of_stream(...) callconv(.c) void;
pub extern fn send_rpc_error_response(...) callconv(.c) void;
```

## Why it matters

In Zig, `pub extern fn` defaults to C calling convention, so this is **not a bug today**. However:

- It is a maintenance hazard — a future reader may not know which functions have been audited for ABI correctness
- It is inconsistent with the established pattern in the same file
- Given that an ABI mismatch on this exact boundary just caused a production GPF (#743), explicit annotations serve as documentation that the ABI contract was intentionally verified

## Fix

Add `callconv(.c)` to all `pub extern fn` declarations in `pkgs/network/src/ethlibp2p.zig` that are missing it:

```zig
pub extern fn create_and_run_network(...) callconv(.c) void;
pub extern fn wait_for_network_ready(...) callconv(.c) bool;
pub extern fn publish_msg_to_rust_bridge(...) callconv(.c) void;
```

While at it, audit other FFI boundary files (e.g. `pkgs/xmss/`, `pkgs/key-manager/`) for the same inconsistency.

## References

- PR #743 (root cause: missing `extern "C"` on Rust side)
- `pkgs/network/src/ethlibp2p.zig` lines ~915–947

Start in pkgs/network/src/ethlibp2p.zig around lines 915–947 and review every pub extern fn declaration against the existing callconv(.c) pattern. Then audit the FFI boundary files under pkgs/xmss/ and pkgs/key-manager/ for the same inconsistency. Done means the missing declarations have explicit callconv(.c) annotations and the affected FFI files are consistently documented.

Written by the indexing model from the issue text.