Measure each item its own way; never compare a partial size (hq ADR 0233)

A walk over a large library every hour loads the array that protects it. An item now says how it
is measured — a bounded daily walk, a dataset's counters, or its top level only — and a size that
is a lower bound is kept as such and never read as a shrink.

internal/inventory/migrations/0072-a-module-declares-the-data-it-holds.sql

@@ -38,6 +38,8 @@ create table data_item (
     last_backup  timestamptz,
     -- What the holder could not measure, when it could not: the path gone, a walk refused.
     measure_error text,
+    -- What the newest size is: exact, a dataset's whole size, partial (a lower bound) or none.
+    precision     text,
     -- The redundant storage it is on, as the holder read it: zfs, md or btrfs, which pool or device,
     -- whether it is healthy, and what it said.
     redundancy_kind    text,
@@ -53,7 +55,8 @@ create table data_item (
 );
 
 -- One row per measurement kept, at most one an hour per item, for ninety days: what a shrink is
--- read against.
+-- read against. Only a comparable size is kept: exact, or a dataset's own counters — never a partial
+-- walk's lower bound.
 create table data_reading (
     machine     text        not null,
     module      text        not null,