Add Rust coding conventions and self-explanatory commenting guidelines; update dependencies and version in Cargo files; enhance OpenBSD support in installation and documentation; improve TCP socket configuration and testing

docs/OPENBSD.en.md

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+# Telemt on OpenBSD (Build, Run, and rc.d)
+
+This guide covers a practical OpenBSD deployment flow for Telemt:
+- build from source,
+- install binary and config,
+- run as an rc.d daemon,
+- verify basic runtime behavior.
+
+## 1. Prerequisites
+
+Install required packages:
+
+```sh
+doas pkg_add rust git
+```
+
+Notes:
+- Telemt release installer (`install.sh`) is Linux-only.
+- On OpenBSD, use source build with `cargo`.
+
+## 2. Build from source
+
+```sh
+git clone https://github.com/telemt/telemt
+cd telemt
+cargo build --release
+./target/release/telemt --version
+```
+
+For low-RAM systems, this repository already uses `lto = "thin"` in release profile.
+
+## 3. Install binary and config
+
+```sh
+doas install -d -m 0755 /usr/local/bin
+doas install -m 0755 ./target/release/telemt /usr/local/bin/telemt
+
+doas install -d -m 0750 /etc/telemt
+doas install -m 0640 ./config.toml /etc/telemt/config.toml
+```
+
+## 4. Create runtime user
+
+```sh
+doas useradd -L daemon -s /sbin/nologin -d /var/empty _telemt
+```
+
+If `_telemt` already exists, continue.
+
+## 5. Install rc.d service
+
+Install the provided script:
+
+```sh
+doas install -m 0555 ./contrib/openbsd/telemt.rcd /etc/rc.d/telemt
+```
+
+Enable and start:
+
+```sh
+doas rcctl enable telemt
+# Optional: send daemon output to syslog
+#doas rcctl set telemt logger daemon.info
+
+doas rcctl start telemt
+```
+
+Service controls:
+
+```sh
+doas rcctl check telemt
+doas rcctl restart telemt
+doas rcctl stop telemt
+```
+
+## 6. Resource limits (recommended)
+
+OpenBSD rc.d can apply limits via login class. Add class `telemt` and assign it to `_telemt`.
+
+Example class entry:
+
+```text
+telemt:\
+    :openfiles-cur=8192:openfiles-max=16384:\
+    :datasize-cur=768M:datasize-max=1024M:\
+    :coredumpsize=0:\
+    :tc=daemon:
+```
+
+These values are conservative defaults for small and medium deployments.
+Increase `openfiles-*` only if logs show descriptor exhaustion under load.
+
+Then rebuild database and assign class:
+
+```sh
+doas cap_mkdb /etc/login.conf
+#doas usermod -L telemt _telemt
+```
+
+Uncomment `usermod` if you want this class bound to the Telemt user.
+
+## 7. Functional smoke test
+
+1. Validate service state:
+
+```sh
+doas rcctl check telemt
+```
+
+2. Check listener is present (replace 443 if needed):
+
+```sh
+netstat -n -f inet -p tcp | grep LISTEN | grep '\.443'
+```
+
+3. Verify process user:
+
+```sh
+ps -o user,pid,command -ax | grep telemt | grep -v grep
+```
+
+4. If startup fails, debug in foreground:
+
+```sh
+RUST_LOG=debug /usr/local/bin/telemt /etc/telemt/config.toml
+```
+
+## 8. OpenBSD-specific caveats
+
+- OpenBSD does not support per-socket keepalive retries/interval tuning in the same way as Linux.
+- Telemt source already uses target-aware cfg gates for keepalive setup.
+- Use rc.d/rcctl, not systemd.