feat: publish-ready overhaul — lints, tests, docs, CI, fuzzing, mutant testing

- Enable clippy pedantic + nursery lints; fix all warnings
- Add config validation (SF, CR, frequency range checks)
- Zeroize passphrase intermediates after key derivation
- Convert to lib+bin crate for fuzz target accessibility
- Replace Vec with VecDeque for O(1) log/bucket rotation
- Add TUI minimum terminal size guard

Testing:
- 145 unit + property tests (was 15)
- 6 proptest groups (frame roundtrip, hash, SNR, air time, peer list, padding)
- 2 cargo-fuzz targets (gossip frame decode, peer list decode)
- cargo-mutants: 0 missed, 241 caught, 54 unviable

Documentation:
- README with prerequisites, quick start, config reference, architecture
- Module-level and item-level rustdoc on all public items
- LICENSE (MIT), rustfmt.toml, .gitignore

Infrastructure:
- GitHub Actions CI (fmt, clippy, test, audit)
- cargo-mutants config (.cargo/mutants.toml)
- justfile for common tasks

.cargo/mutants.toml

@@ -0,0 +1,55 @@
+# cargo-mutants configuration
+# Run with: cargo mutants
+# Install with: cargo install cargo-mutants
+
+# Exclude entire files that are untestable (main requires full app, setup requires interactive stdin)
+exclude_globs = [
+    "src/main.rs",
+    "src/setup.rs",
+]
+
+# Exclude specific functions that require hardware, network, terminal, or full async infrastructure
+exclude_re = [
+    # Equivalent mutants: Instant arithmetic never produces exact equality,
+    # so `< ttl` and `<= ttl` behave identically in practice.
+    "replace < with <= in DedupCache",
+    "replace < with <= in decode_peer_list",
+    # rate_limit.rs: equivalent mutant (secs is always > 0 from lora_air_time)
+    "replace > with >= in RateLimiter::from_radio_config",
+    # radio.rs: hardware-dependent functions
+    "radio_loop",
+    "connect_and_run",
+    # gossip.rs: async network functions
+    "Gossip::new",
+    "Gossip::shutdown",
+    "spawn_accept_loop",
+    "handle_incoming",
+    "spawn_receiver",
+    "spawn_broadcaster",
+    "spawn_dht_heartbeat",
+    "dht_read_and_join",
+    "dht_publish",
+    "dht_get_peers",
+    # router.rs: async event loop
+    "^src/router\\.rs:.* replace run ",
+    "^src/router\\.rs:.* in run$",
+    # tui.rs: terminal rendering functions
+    "tui::run",
+    "tui_loop",
+    "restore_terminal",
+    "draw_ui",
+    "draw_header",
+    "draw_network",
+    "draw_radio",
+    "draw_stats",
+    "draw_log",
+    "draw_activity",
+    "draw_sparkline_section",
+    "draw_shutdown_overlay",
+    "panel_block",
+    "status_pill",
+    "kv_line",
+]
+
+# Timeout per mutant test (seconds)
+timeout_multiplier = 3.0