feat: report speed in MB/s instead of Mbps

- drop the *8 bit conversion so throughput is MB/s, matching what
  clients like v2rayN display
- rename speed_mbps -> speed_mbs across speed/report/commands
- narrow the ranking bucket to 0.1 MB/s so slow (~0.1-0.3 MB/s) nodes
  still separate by speed instead of collapsing into one bucket
- update table/CSV/alias labels, color thresholds, --min-speed help,
  and README

src/cli.rs

@@ -71,7 +71,7 @@ pub(crate) struct AutoArgs {
     #[arg(long, default_value_t = 0.0)]
     pub(crate) lat_max: f64,
 
-    /// Quality gate: drop IPs slower than this (Mbps). 0 = off.
+    /// Quality gate: drop IPs slower than this (MB/s). 0 = off.
     #[arg(long, default_value_t = 0.0)]
     pub(crate) min_speed: f64,
 
@@ -199,7 +199,7 @@ pub(crate) struct SpeedArgs {
     #[arg(long, default_value_t = 10_000_000)]
     pub(crate) bytes: u64,
 
-    /// Reject IPs slower than this (Mbps). 0 = keep all.
+    /// Reject IPs slower than this (MB/s). 0 = keep all.
     #[arg(long = "min", default_value_t = 0.0)]
     pub(crate) min_speed: f64,