Measure the borrow, rather than asserting it bounds the disk

The claim that hydration-as-a-borrow makes peak disk the working set
rather than the library was so far an argument. `--example vfs_cycle`
runs it: 100 photographs of 25 MB, 90 dehydrated, a pass over all of
them through the real engine.

Peak 275 MB — the resting set plus one photograph — against 2,500 MB
had the pass simply fetched everything. Back to 250 MB afterwards, and
all ten files the user already kept still there, which is the half of
the contract that matters more.

Recorded in docs/storage.md §6.3 and ARCH §9.0a, because a bound argued
from a number nobody measured is one that gets quietly lost.

docs/traceability.md

@@ -9,7 +9,7 @@ Denominators are parsed from [`requirements.md`](requirements.md) at run time, n
 
 | Metric | Value |
 |---|---|
-| Source files scanned | 286 |
+| Source files scanned | 287 |
 | TRACES tags found | 840 |
 | Requirements defined | 179 |
 | Requirements covered | 107 |