sems-stats: port to safe Rust, add -q switch

[hecko]

Summary

• Re-implement sems-stats in safe Rust (#![forbid(unsafe_code)]), std-only, no external crates. Lives in the existing Cargo workspace at apps/monitoring/tools/sems-stats/ alongside the other Rust monitoring tools.
• Feature parity with the C++ client: same flags (-s -p -c -t -h), same defaults (127.0.0.1:5040, calls, 5s), same human-readable framing (sending '...\n' to host:port / received:\n<reply>), same exit-code semantics.
• Add -q / --quiet switch that prints only the server reply with a single trailing newline — useful for shell captures ($(sems-stats -q -c calls)) and Zabbix UserParameters.
• The C++ sources (core/plug-in/stats/query_stats.cxx) are untouched. The C++ target still builds as a reference / fallback; the install step only runs when the Rust toolchain is unavailable, so there is no binary collision at $SEMS_EXEC_PREFIX/sbin/sems-stats.

Build integration

• Added sems-stats to apps/monitoring/tools/Cargo.toml workspace members.
• Added it to SEMS_MONITORING_BINARIES in apps/monitoring/CMakeLists.txt so the existing cargo build / install plumbing picks it up.
• Exposed Rust-toolchain detection as a CACHE INTERNAL (SEMS_HAS_RUST) so core/plug-in/stats/CMakeLists.txt can skip its install() when the Rust port is being shipped.
• Registered cargo test -p sems-stats with CTest via add_test, so ctest -V exercises the crate.

Tests

• 12 unit tests in src/lib.rs covering parse_args (defaults, every flag combination, every error variant), resolve_server, trim_reply (including the trailing-NUL semantic matching the C++ server's wire format), and the usage text.
• 3 integration tests in tests/udp_roundtrip.rs that spin up a local UDP responder on 127.0.0.1:0 and exercise:
  • happy-path request/reply,
  • a command with a space (set_loglevel 1) to confirm argv isn't tokenised,
  • the receive-timeout path (200 ms budget).
• cargo test -p sems-stats --release → 15 passed, 0 failed.

Test plan

• cmake -S . -B build && cmake --build build succeeds on a host with a recent Rust toolchain and produces apps/monitoring/tools/target/release/sems-stats.
• ctest -V in the build directory runs the cargo tests and they pass.
• On a host without cargo/rustc, the C++ sems-stats is still installed (existing behaviour preserved).
• sems-stats -c calls prints the same sending ... / received: ... framing as the C++ version against a live SEMS instance.
• sems-stats -q -c calls prints a single line Active calls: N.
• RPM build (Dockerfile-rhel{8,9,10}) still lands a sems-stats in the package at the same path.

Refs #438 (indirectly — this is the Zabbix-UserParameter-friendly output path that issue will rely on).

https://claude.ai/code/session_01BDutRs8JU8stspu6kpCQpd