feat(net): add {linkSpeed} format replacement

Adds an ethernet link speed ({linkSpeed}) read from
/sys/class/net/<iface>/speed, plus a skip-decimal option on pow_format
to drop a trailing .0. Rebased onto master's store-based network
formatting; existing compact bandwidth args updated for the new
pow_format signature.

man/waybar-network.5.scd

@@ -200,6 +200,8 @@ Addressed by *network*
 
 *{rx_bitrate}*: Link receive bitrate (e.g., 866.7 Mb/s).
 
+*{linkSpeed}*: Ethernet link speed.
+
 *{icon}*: Icon, as defined in *format-icons*.
 
 # EXAMPLES