pre-flight: bandwidth test + persistent config

First-run host launch now offers a one-time upstream measurement
against speed.cloudflare.com/__up via ureq (~5 MB POST, ~5s). The
result lives at ~/.config/pixelpass/config.toml under [bandwidth]
and feeds the default --max-viewers calculation on subsequent runs.

Sticky semantics for the dialog:
- Unmeasured: first-run prompt (Run / Skip)
- Measured / Skipped: silent — never re-prompts
- Failed: ask again on next launch (Retry / give up → Skipped)

`pixelpass --reconfigure` re-runs the test unconditionally for users
whose connection has changed (new ISP, moved house, etc.).

--max-viewers is now Option<u32>. When unset, host startup loads the
saved measurement, runs recommended_max_viewers(safe_mbps, bitrate),
and surfaces the source in the banner: "max viewers : N (auto: X.X
Mbps measured upstream)" — or user-specified / default fallback.

User verified end-to-end on 2026-05-21 16:54 EDT: first-run dialog,
skip path, run path, --reconfigure refresh, and banner integration
all work as expected.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/common/bandwidth.rs

@@ -0,0 +1,73 @@
+//! One-shot upstream bandwidth measurement against Cloudflare's open
+//! speed-test endpoint. POST a fixed payload, time it, derive Mbps.
+//!
+//! Run via `tokio::task::spawn_blocking` from async contexts — ureq is a
+//! blocking client and we don't want to wedge the tokio runtime during
+//! the test.
+
+use anyhow::{Context, Result};
+use std::time::{Duration, Instant};
+
+const ENDPOINT: &str = "https://speed.cloudflare.com/__up";
+const PAYLOAD_BYTES: usize = 5 * 1024 * 1024; // 5 MiB
+const HTTP_TIMEOUT: Duration = Duration::from_secs(30);
+/// Multiplier applied to the raw measurement. TCP slow-start, ramp-up, and
+/// real-world contention all mean a one-shot upstream test slightly
+/// overestimates sustainable throughput; clamp to 80% for headroom.
+const SAFETY_FACTOR: f64 = 0.80;
+
+/// Result of a successful measurement.
+#[derive(Debug, Clone)]
+pub struct Measurement {
+    /// Raw measured throughput in megabits per second.
+    pub raw_mbps: f64,
+    /// `raw_mbps * SAFETY_FACTOR` — the value to use when sizing things.
+    pub safe_mbps: f64,
+    /// How long the upload took.
+    pub elapsed: Duration,
+}
+
+/// Blocking upload-speed test. Call from a `spawn_blocking` task.
+pub fn measure_upstream_blocking() -> Result<Measurement> {
+    let payload = vec![0u8; PAYLOAD_BYTES];
+    let agent = ureq::Agent::config_builder()
+        .timeout_global(Some(HTTP_TIMEOUT))
+        .build()
+        .new_agent();
+
+    let start = Instant::now();
+    let response = agent
+        .post(ENDPOINT)
+        .content_type("application/octet-stream")
+        .send(&payload[..])
+        .context("upload request to Cloudflare failed")?;
+    let elapsed = start.elapsed();
+
+    let status = response.status();
+    if !status.is_success() {
+        anyhow::bail!("Cloudflare returned HTTP {status}");
+    }
+
+    let bits = (PAYLOAD_BYTES as f64) * 8.0;
+    let seconds = elapsed.as_secs_f64().max(0.001);
+    let raw_mbps = bits / seconds / 1_000_000.0;
+    let safe_mbps = raw_mbps * SAFETY_FACTOR;
+
+    Ok(Measurement {
+        raw_mbps,
+        safe_mbps,
+        elapsed,
+    })
+}
+
+/// Convert a safe-upstream Mbps figure plus the host's per-viewer bitrate
+/// (kbps for video, ignoring audio + protocol overhead which we account for
+/// via SAFETY_FACTOR) into a recommended viewer count. Floors to at least 1.
+pub fn recommended_max_viewers(safe_mbps: f64, bitrate_kbps: u32) -> u32 {
+    let per_viewer_mbps = (bitrate_kbps as f64) / 1000.0;
+    if per_viewer_mbps <= 0.0 {
+        return 1;
+    }
+    let n = (safe_mbps / per_viewer_mbps).floor();
+    if n < 1.0 { 1 } else { n as u32 }
+}