fix(adr-115): CI green — example feature-gate + mosquitto allow_anon + bench numbers

## Two CI failures on PR #778 fixed

### 1. Rust Workspace Tests (E0601: `main` not found in mqtt_publisher)

Default `cargo build --workspace` compiles examples without forwarding
`--features mqtt`. The example had a crate-level `#![cfg(feature =
"mqtt")]` so the entire file evaporated, leaving zero `main`. Now
provides a stub `main` when the feature is off (prints a hint and
exits 2), and gates the real implementation behind `#[cfg(feature =
"mqtt")]` per-item.

Local verification:
  cargo check --no-default-features --examples → clean

### 2. mqtt-integration (mosquitto never became reachable)

`eclipse-mosquitto:2.x` rejects anonymous connections by default and
GH Actions `services:` containers don't easily support volume-mounting
a custom config. Removed the service container and start mosquitto
manually in a step with an inline `allow_anonymous true` listener on
port 11883. Same wire shape, no auth (CI tests protocol behaviour,
not security — production uses mTLS per ADR §3.9).

## Benchmark numbers captured (`docs/integrations/benchmarks.md`)

Ran `cargo bench --features mqtt --bench mqtt_throughput` locally:

| Hot path                              | Measured | Target | Better by |
|---------------------------------------|----------|--------|-----------|
| state::event_fall encode              | 259 ns   | <2 µs  | 7.7×      |
| rate_limiter::allow_first             | 49.7 ns  | <100 ns| 2×        |
| rate_limiter::allow_within_gap        | 62.1 ns  | <100 ns| 1.6×      |
| privacy::decide_hr_strip              | 0.24 ns  | <50 ns | 208×      |
| privacy::decide_presence_keep         | 0.24 ns  | <50 ns | 208×      |
| semantic::bus_tick_all_10_primitives  | 717 ns   | <10 µs | 14×       |

At 1 Hz publish rate per node, the entire ADR-115 hot path costs
~1 µs per node per tick on commodity hardware. A Cognitum Seed
hosting 100 nodes would burn 100 µs/sec — 0.01% load floor. Memory:
~30 KB total FSM state for 10 primitives × 100 nodes.

The numbers exceed every target by ≥1.6×, several by 100×+. No need
to optimise further for v0.7.0.

Refs #776, PR #778.

Co-Authored-By: claude-flow <ruv@ruv.net>

v2/crates/wifi-densepose-sensing-server/examples/mqtt_publisher.rs

@@ -20,15 +20,35 @@
 //! the active edit surface of the parallel ADR-110 agent — see
 //! [[feedback-multi-agent-worktree]]).
 
-#![cfg(feature = "mqtt")]
+// The full example body needs the `mqtt` feature (rumqttc, publisher::spawn,
+// etc.). When the feature is off we provide a stub `main` so the example
+// still compiles cleanly during a default `cargo build --workspace` —
+// otherwise CI fails with E0601 (`main function not found`) on every PR
+// that touches the workspace, even ones unrelated to ADR-115.
+#[cfg(not(feature = "mqtt"))]
+fn main() {
+    eprintln!(
+        "This example requires --features mqtt. Re-run with: \n  \
+         cargo run -p wifi-densepose-sensing-server --features mqtt \
+         --example mqtt_publisher -- --mqtt"
+    );
+    std::process::exit(2);
+}
 
+#[cfg(feature = "mqtt")]
 use std::sync::Arc;
+#[cfg(feature = "mqtt")]
 use std::time::Duration;
 
+#[cfg(feature = "mqtt")]
 use clap::Parser;
+#[cfg(feature = "mqtt")]
 use tokio::sync::broadcast;
+#[cfg(feature = "mqtt")]
 use tracing::info;
+#[cfg(feature = "mqtt")]
 use wifi_densepose_sensing_server::cli::Args;
+#[cfg(feature = "mqtt")]
 use wifi_densepose_sensing_server::mqtt::{
     config::MqttConfig,
     publisher::{spawn, OwnedDiscoveryBuilder},
@@ -36,6 +56,7 @@ use wifi_densepose_sensing_server::mqtt::{
     state::VitalsSnapshot,
 };
 
+#[cfg(feature = "mqtt")]
 #[tokio::main]
 async fn main() -> Result<(), Box<dyn std::error::Error>> {
     tracing_subscriber::fmt::init();