Space reconciliation passes by ten times the last pass's time (closes #250)

The reconciliation pass reads a row and stats a file for every cached
file, and it ran on every eviction tick, so on a large cache a pass
longer than the tick was followed by the next one at once. It still runs
at startup and is considered on each tick, but it now starts no sooner
than ten times as long after the last pass ended as that pass took.
Eviction and the removal of expired negative cache rows still run on
every tick. The comments that bound the drift, and README.md, give the
new bound.

Model: opus-5-5

TODO.md

@@ -30,6 +30,9 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-18 the next reconciliation pass waits at least ten times as long as
+  the last one took, so passes on a large cache no longer run back to back
+  (closes #250).
 - 2026-10-18 a `blocked_networks` entry in the IPv4-mapped form refused nothing;
   `::ffff:10.0.0.0/104` is now read as `10.0.0.0/8`, and such an entry shorter
   than `/96` aborts startup (closes #301).