feat: v0.3.0 stabilization -- systemd, tests, API docs

- Bump version 0.1.0 -> 0.3.0
- Add systemd service unit (config/s5p.service) and install-service
  Makefile target
- Add CLI argument parsing tests (tests/test_cli.py, 27 tests)
- Expand protocol tests with SOCKS5/4/HTTP handshake, error, and auth
  coverage (tests/test_proto.py, 30 tests)
- Add full API reference to docs/USAGE.md with response schemas for
  all GET/POST endpoints
- Update INSTALL.md, CHEATSHEET.md with systemd section
- Update ROADMAP.md, TASKS.md for v0.3.0

ROADMAP.md

@@ -1,6 +1,6 @@
 # s5p -- Roadmap
 
-## v0.1.0 (current)
+## v0.1.0
 
 - [x] SOCKS5 server (CONNECT command)
 - [x] Proxy chaining (SOCKS5, SOCKS4/4a, HTTP CONNECT)
@@ -32,9 +32,13 @@
 - [x] Named proxy pools with per-listener assignment (`proxy_pools:`)
 - [x] MITM source filter (`mitm: true/false` on pool sources)
 
-## v0.3.0
+## v0.3.0 (current)
 
 - [x] SOCKS5 server authentication (username/password)
+- [x] Systemd service unit
+- [x] CLI test coverage
+- [x] Protocol test coverage (SOCKS5/4/HTTP handshakes)
+- [x] API documentation (full response schemas)
 - [ ] UDP ASSOCIATE support (SOCKS5 UDP relay)
 - [ ] BIND support
 - [ ] Chain randomization (random order, random subset)
@@ -42,6 +46,4 @@
 ## v1.0.0
 
 - [ ] Stable API and config format
-- [ ] Comprehensive test suite with mock proxies (integration tests done)
-- [ ] Systemd service unit
 - [ ] Performance benchmarks