Carry the power draw to the bar over the bus, not through a file (hq ADR 0260)

Modules talk over the bus. The power module's bundle puts each fresh reading of its own
sampler in its state draw, keyed by the machine; the bar's block shows that state, which
the bar's bundle watches under the read the mesh grants it. A new catalogue check refuses
a module naming another module's path under /run or /var/lib.

modules/i3status-rust/README.md

@@ -41,6 +41,14 @@ contributes it to `node-bar` as data that says what it shows, and this module re
 - **Where it applies:** a contribution may name a capability the machine must report. The `power`
   module's battery block names `battery` and its power-draw block names `power-meter`, so the laptop's
   bar gets both and a desktop only the power draw.
+- **A value from the bus:** a block that `shows: state` names another module's state, such as
+  `power.draw`. The mesh grants this module the read, because a module on its machine offers it; the
+  manifest names no other module. This module's bundle watches it and leaves the value's text in the
+  account's runtime directory (`i3status-rust/values/`), and the block runs `~/.local/bin/i3status-value`,
+  which prints it, or nothing when it is more than 30 s old, so the block hides. No file of another
+  module is read: modules talk over the bus.
+- **The battery** is i3status-rust's own battery block, which reads the kernel's power-supply class
+  itself: the bar reading the machine, not another module.
 
 Still left out: