Follow bar values so a dead watch speaks, date them by their reading, and widen the path check (hq ADR 0260)

A watch that died left the block silent, a removed block kept its value, and a value
frozen at its source looked fresh because its file was new. The follower now rereads a
silent value and says so, drops removed blocks, and dates each file by the reading's
own time. The path check takes /etc and /usr/local/lib in, ignores what a container sees
inside itself, and lists the five findings standing today, which may only go.

modules/power/README.md

@@ -58,7 +58,8 @@ permission is loosened. Two parts of this module carry the reading:
 1. the process `mesh-power-draw` (`power draw`, as root) samples every 5 s and leaves the averaged rate
    in `/run/mesh-power/draw`, this module's own directory, read by nothing but this module;
 2. the module's bundle, which the runtime launches, puts each fresh reading in its state `draw`, keyed by
-   the machine's name: `{"text", "watts", "measures", "at"}`. A value lives 30 s (`ttl-seconds`).
+   the machine's name: `{"text", "watts", "measures", "at"}`. A value lives 30 s (`ttl-seconds`). The
+   state is `per-machine`: each machine's instance is granted its own machine's key and no other.
 
 **State, not events, by bus load.** A reading is the current value and is replaced every five seconds;
 nobody needs the ones before. A key per machine keeps one value, so the bus holds at most one reading

modules/power/module.json

@@ -23,7 +23,8 @@
   "state": [
     {
       "name": "draw",
-      "ttl-seconds": 30
+      "ttl-seconds": 30,
+      "per-machine": true
     }
   ],
   "tools": [